Seems to me... If you have a 9.9-70 horse go buddy tail and it turns the same rpm as the next guy running the same prop then the speeds should be relatively the same. I know there is prop slip to factor in but it seems like it should be similar. My boat has always ran at 4k. With the mikuni I can now spin way on up there. 4k is and always has been 25/26. Now 4800 = 30. This is with a wore out big blade. I am going to regear and get a new hammer and see where I am at but it seems off that a stock 37 will run 30 mph. Is my prop that bad or are these folks rounding up a lot.

Sounds to me like your prop is that bad.
Your boat should move along a little better with those rpm's IMO.

If you know your gearing and rpm and speed, you can use a formula to figure the slip of a given prop and effiencey of a hull. Here is an example I've found to be pretty close. My motor will turn a Hammer 12.75D x 11P prop at 4450 rpm with a 1.24 gear ratio 38/47 gears. RPM divided by 1.24 = 3588 x 11p= 39468. 39468 divided by 1056= 37.3 mph. MInus 10% slippage= 3.73. 37.3 minus 3.73 =33.57 mph. Slippage percent can be more or less depending on hull shape and weight and load. Prop Diameter, pitch and cupping will affect slippage percent also. My rig has been police radared, GPS measured with a Garmin Novi, a Megellan GPS unit, and a smart phone app. All were within 1 mph +or- of the formulas mph. So, my rigs slippage percentage must be around 10% proving the formula is pretty close. Confused yet?

Good example denny, thanks

Nanny, with a 45tooth lower gear on my gt, I can sit at 33 at 4600 rpms with two people. Speeds will be similar with similar prop and rpms, but the weight your pushing, the depth your boat sits, how much prop is biting the water, amount of cup, etc etc all factor in. So while the formula denny posted is applicable, the variables won't be the same for a 1748 vs 1848, etc. That's why there are such differences in speed. Add in that guys exaggerate because it's cool to be faster than the next guy, and that will explain some of the inconsistencies.

That is what I was looking for. My boat is heavy but I did not think it was that heavy. I currently have a 48 bottom gear. I have found if I lower my prop I get a faster speed. If I have the typical rooster tail I lose about 3 mph. Both of the props I have are wore out. I will do some testing when my new one comes in.

Will def make a difference.

Raising your prop, will cause it to loose bite being that one of the blades is mostly out of the water. The only way to combat this is to run a prop with a lot of cup. Teague Racing Marine gave me the formula to help me prop my 29' performance boat and set the drive height. Running that rig with prop near surface, we had to go to a 4 Blade wheel in order to keep it from blowing out. Theroy was that having extra blade in water at all times kept prop from ventilating and loosing bite.I've always wondered why a prop manufacture hasn't tested a 4bld wheel for a surface drive mud motor. Some of the Arnsen surfacedrives on performance boats have 5bld wheels..?? Probably trying to push to much boat with to little HP.