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 Shaft length question 
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Post Shaft length question
Not sure if this is the right spot for this question but here it goes.

I’m getting ready to start building a surface drive. I’ve did a bunch of research before on the subject of the shaft length, from bottom pulley center to center of prop. I just wanted to see if anyone could pull a tape and just get an approximate length on that to make sure I’m not way off. I’m shooting for a design real close to a copperhead 23 but from pictures I’m thinking I might be a fuzz longer than is needed. Any help or comments on the overall build process are welcomed. Thanks guys


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Thu Feb 03, 2022 8:13 am
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Post Re: Shaft length question
Build to suit the boat....

It needs to trim down enough to get the whole prop below the hull a few inches and
the housing assembly has to be above the hull an inch or two at that full trim....

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Sat Feb 05, 2022 8:45 pm
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Post Re: Shaft length question
Thanks Giga. I am designed in that fashion now. Housing stays above while the prop drops plenty below to dig if needed. Got the bearings ordered and will be ordering a new vanguard soon


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Sat Feb 05, 2022 8:49 pm
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Post Re: Shaft length question
Have you thought much about gear reduction and prop pitch and deign ???

9" is legal in the reserves - slower but digs well...

With no reduction you would be about right on a 9" 6 pitch LT prop....

With 1.4 to 1 or 1.7 to 1 - an 11' 12 pitch might work like the GD SD 3 blade...

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Sat Feb 05, 2022 8:56 pm
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Post Re: Shaft length question
If I remember correctly the oldest GDSD 23 had 1.5 to 1 and turned a 10" 2 blade 10 pitch

Later they used more reduction like 1.7 to 1 and turned the bigger 11" 3 blade 12 pitch

Or something like that - bet if you called them they could set you right....

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Sat Feb 05, 2022 9:05 pm
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Post Re: Shaft length question
On a standard 35 hp - these used to be the correct reduction and pitch for various mfgs...

PD uses a 1.5 to 1 reduction and 12 pitch
GD uses a 1.4 to 1 reduction and 11 pitch
GT uses a 1.2 to 1 reduction and 10 pitch

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Sat Feb 05, 2022 9:11 pm
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Post Re: Shaft length question
Yea, I’ve considered both. At this point I was going with a 9x6 prop. Two blade backwater LT as I’ve been using them a while on my longtails and they’ve done well for me. As for the gearing I’m not sure how to spell it out so bear with me. My plan was to go with a setup that put the prop turning 1.25 times for every turn at the crank. Is that backwards?


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Sat Feb 05, 2022 9:20 pm
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Post Re: Shaft length question
Yes

You need to reduce rpms at the prop to turn a bigger pitch....

Look at the reduction ratios above for a 35hp, the same reductions are also used with a 40hp....

You gain torque at lower speeds and the bigger pitch ends up giving you the speed once it
begins to "surface" and the tips come up above water level...

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Mon Feb 07, 2022 5:44 am
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Post Re: Shaft length question
Ok I will have to rethink that then. So essentially, because you’ll be running with prop out of water at WOT you want that bigger blade/pitch to keep pushing


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Mon Feb 07, 2022 6:17 am
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Post Re: Shaft length question
The fastest boats in the world run over 200mph with a "surfacing prop" designed
to provide thrust with only half of the prop below water level.....

LOL - it all become a mute point if you can't get the boat on plane, but yes the
whole reason these little motors can use a 10 pitch or higher 12" blade is that
the props are designed to "surface" where only half the prop is below the water....

With sufficient torque these props also plow out of a mud hole by being fully
submerged in the water and mud.... That is why a gearing reduction is needed
to get the hull started and as it makes enough speed to plane the hull it gets
elevated and begins surfacing to reduce drag and increase both engine rpm
and boat speeds....

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Mon Feb 07, 2022 8:27 am
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Post Re: Shaft length question
If you are going to run a 9 x 6 pitch - you could use a 1 to 1 ratio where
prop rpm and engine rpms are the same.... But if you want more than
about 20mph - you need the bigger pitch prop designed to "surface" and
an appropriate to 23hp ratio to reduce prop rpm to get enough torque to
dig thru a bit of mud and get the hull on plane...

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Mon Feb 07, 2022 8:32 am
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Post Shaft length question
Giga I really appreciate all the info man. Helped me out a bunch. I just got off the phone with Jake at backwater. He confirmed everything your saying as well. They have a 10x10 prop I’ll go with and use somewhere around 1.2 : 1 gearing. Does that sound about right to you?


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Mon Feb 07, 2022 8:36 am
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Post Re: Shaft length question

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Mon Feb 07, 2022 6:59 pm
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Post Re: Shaft length question
If you want to be stupidly scientific....

Take max engine rpm - account for gearing reduction to prop - convert the number of
inches per revolution (the pitch) - then convert all those inches to MPH - finally take a 10%
deduction for prop slippage and you have max possible speed for that setup.....

Real world - a light load might get 25mph with a 23 hp.... Shoot for that mathematically....


I saw my buddy run a super light GD hull with the 23 GDSD original 10x10 setup at about
that max speed on a 10x10 prop with that 1.5 to 1 reduction.... That lil skiff was also pretty
bad ass with 2 guys and ran about 21mph with two 180 lb guys.... Slower with me in there
but I am more like 230lbs - LOL

Which is why I moved from the 23 LT to the 35 GDSD... As my buddies are all as heavy as I am...

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Mon Feb 07, 2022 7:33 pm
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Post Shaft length question
Appreciate it Giga. I did put a call in with Coco. He was not any help in the least. As a matter of a fact he went out of his way to inform me he was there to help customers of his product only. Took him as long to say that spiel as it would have to just have answered the question. Anyhow, from what your saying it seems like a kind of balancing act between just enough reduction to get onto plane but keeping that reduction minimal to have the speed after your up. So a 1:1 would be the fastest if you can get on plane whereas a 1.5:1 would plane a larger load but won’t have as much top end

I’ve got a gear set I’m looking at now that’s 1.43:1. I may be going with that.


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Tue Feb 08, 2022 3:48 pm
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Post Re: Shaft length question
Figured I’d update this a bit for anyone interested.

Ended up choosing the 1.57:1 gear ratio pulleys. Figured being toward the torque end would be best considering I’m always hauling a pretty good load.

Will be putting most of this together this weekend but here what I’ve got so far….if I can get pictures to load


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Thu Feb 24, 2022 12:21 pm
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Post Re: Shaft length question
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Thu Feb 24, 2022 12:26 pm
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Post Re: Shaft length question
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Thu Feb 24, 2022 12:27 pm
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Post Re: Shaft length question
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Mon Mar 14, 2022 5:01 pm
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Post Re: Shaft length question
Took it for a water test today. Got a problem somewhere. Maybe someone has an idea. Spins fine out of water and fine when air is mixing but as soon as the prop bites it bogs the motor out and won’t spin up to any kind of rpm. Checked spark plugs to make sure both cylinders were hitting and it was quite obvious they are. I’m not sure what to do other than down size the prop. Seems everything I could find said 1.5:1 with a 10x10 prop is what’s used. Am I missing something?


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Mon Mar 14, 2022 5:05 pm
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Post Re: Shaft length question
Cleaned the carb? What RPM are you getting in the water and out?


Mon Mar 14, 2022 6:19 pm
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Post Re: Shaft length question
Think I figured it out. Simplest of mistakes. My gearing is backwards. Will fix and re test


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Mon Mar 14, 2022 6:36 pm
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Post Re: Shaft length question
Gearing changed improved it but still only hit maybe 10mph. Also, belt walked off the bottom pulley and got ate so I converted to chain. Also took the gearing to 1.25:1 when I did that. Gained a bit more speed but something still isn’t right. I’m expecting ~20mph and only getting maybe just over 10. Prop pushes real hard then quits pushing and almost seems to be cavitating. Moved up and down and it’s all about the same. Just like it’s loosing bite, but motor seems to be spinning up as it should.

Question, could it be that the boat is so wrong for a SD that it just won’t work correctly? This is a 1738 identical to a go devil. Round chines and where the transom meets the bottom it’s chopped. Built for a longtail.

I’m planning to try it on a 1648 jon and see how that goes and I also ordered a tiny tac to put on the engine just to see about the refs loaded.

Any suggestions?


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Tue Mar 29, 2022 11:53 am
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Post Re: Shaft length question
That boat definitely looks like a GD hull, did kliebert build it?

What RPMs are you at wide open? That's really going to tell a lot, if your spinning 3800 and going 10 mph, there might be a prop issue or your not able to trim down enough to let the prop grab. the free GPS apps for phones will give you an idea of how fast you are going.

Have you ruled out everything with the engine? it's probably fine and will be ruled out if you had a tach to make sure your running wide open.

And maybe I missed it, but did you end up going with a prop from backwater inc, or backwater performance? I cant tell, but the prop almost looks like its a prop for a longtail (backwater inc, sub surface) and not a surface drive prop. That could be your issue.

Can you post a side pic, as level as you can, so that we can see where the prop height is in relation to the bottom of the boat while at the running height? If you search threads where people bought mismatched transoms/motors that will give you a good idea of that issue.


Tue Mar 29, 2022 12:32 pm
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Post Re: Shaft length question
Well, it is a backwater inc prop, 10 x 10, so it may be different? I had talked to Jake there and he seemed to have dived into the SD world a bit and offered that prop even though they don’t list them for sale but that could be the issue. It would be nice if I had anyone around to try a prop from. I’m in WV so I’m very limited. I’m the only one I’ve heard of doing any of this.

The boat is in fact a kliebert. The motor was a 23 go devil longtail and I believe it was a delta waterfowl edition if that makes any difference.

I’ll have to get that tac installed and double check revs but I really think it’s fine in that aspect

How should the prop run in relation to the bottom of the boat? Even? Under? I’m not dragging the housing if that’s what your looking for


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Tue Mar 29, 2022 12:40 pm
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Post Re: Shaft length question
Surface drives generally run 50% in water while wide open. As you start building speed, and your at full throttle, and the prop has lost its bite, you need to be trimming up or down to achieve that sweet spot where the vibration is low and the speed is good.

Here's the link to the operation manual for a GDSD, may give you an idea of how to get the trim right.

https://www.godevil.com/wp-content/uplo ... l-2011.pdf


Tue Mar 29, 2022 1:40 pm
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Post Re: Shaft length question
Circling back around - that 23 on a LT with a 9x6 prop would get your hull on plane
and run about 17mph - solo...

In LT engine and prop at 4000 rpm times 6" = 24,000 inches per minute roughly 20mph mathematically
before friction and prop slippage

At 4000 engine with 1.6 reduction - prop = 2500 rpm times 10" - 25,000 inches per minute
roughly 24 mph before friction and slippage

You still have too much pitch or not enough reduction to get it on plane and match
up to direct drive pitch on a 9x6 LT prop....

Like BigBend said you can "surface the prop" - but first you have to get the hull on plane....

Right now you are stuck just below planing speed with too much drag to enjoy "surfacing the prop"
and the extra speed that brings as the motor revs up higher and higher....

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Wed Mar 30, 2022 8:14 am
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Post Re: Shaft length question
Right I understand what your saying. And I did try the 1.5:1 we talked about and it was slower. It seemed the prop was spinning out without enough speed get me near plaining. I’m going to get the tac setup and check rpms with load when I’ve got time. Real busy lately. I do appreciate you guys trying to help me out


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Wed Mar 30, 2022 8:19 am
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Post Re: Shaft length question
Then - that prop must not be designed for "surfacing"

It could be intended for a 35 LT where it stays buried below the waterline
with a motor with the power to turn it that way.....

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Wed Mar 30, 2022 8:44 am
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Post Re: Shaft length question
Yeah that’s the bad thing. That’s probably the whole issue.


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Wed Mar 30, 2022 10:29 am
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Post Re: Shaft length question
I’m going to send the backwater inc prop back and get a true surface drive prop. Talked to Jake and he confirmed that prop was in fact intended for a longtail.

Question. Are hill marine surface props any good? They’re a good $80 cheaper than a backwater performance prop


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Wed Mar 30, 2022 10:52 am
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Post Shaft length question
Got the tac installed. Looks like I’m spinning ~4200 rpm. But that is without a prop even on the shaft. Will get more numbers with a new prop.

Any recommendations? We really don’t have a whole lot of mud here. Mostly stumps and Lilly pads. The main thing I really need is to carry a load decently. What I saw so far is a hill marine 11x10 for $200 and a backwater performance 11x10 for $260


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Wed Mar 30, 2022 9:02 pm
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Post Re: Shaft length question
Little update here. Got the Hill Marine 11x12 surface prop. That did the trick. Ran 18–19 with just me and about 15 with me and my large buddy. Those speeds seem a little slower than I was expecting my gearing at the moment is 1.25:1. The motor will still power through and get us moving in a few feet. With just me it takes right off. I do think I need to go back to 1.5:1 to get that good low end power. Can I expect to be slower?

1 guy. 18 mph. 3600 rpms running
2 guys. 15 mph. 3400 rpm max 3000-3200 typically


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Thu Apr 07, 2022 5:07 pm
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Post Re: Shaft length question
I had a 23hp GDSD on a 1648, got slightly higher speeds (not much) and ran around 3800-3900 RPMs on that rig WOT. That was with a GD 2 blade prop for the 23.

With some more stick time you will probably see a little increase as you get your prop depth dialed in.

Cant say much about the gear ratio change, I don't have any experience with playing with that.

Are you still running a chain in place of the belt? After you get everything else figured out, I would focus on getting back to that thing being belt driven. There have been lots of fly by night companies that tried to use the chain drive and non have lasted.


Fri Apr 08, 2022 7:57 am
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Post Shaft length question
Yeah I’d like to go back to a belt drive but I’m not sure why it walked off the pulley. Has to be slightly misaligned. Just makes it hard to trust that. I see no reason chain won’t work, and it’s no louder than the belt was. I can trust it not to walk off the cog and it’s not getting much tougher than chain. It’s single strand ansi 40 series. Motorcycle chain. I figured if a motorcycle can run around in the dirt and muck using chain then why can’t I use it in the clean housing and get good life out of it. Anyhow, just my thoughts. I think I might run this gearing a while and see how it does. Will try the 1.5:1 at some point.


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Fri Apr 08, 2022 8:05 am
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Post Re: Shaft length question
Built an exhaust today

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Sat Apr 09, 2022 5:39 pm
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