I don't understand, if it ran 62 and now runs 70, he gained 8 mph. That's not nothing.

Remember that 8 years ago the government had the GPS selective inaccuracy and the 72 miles an hour reading could have been while the system was purposely inaccurate. I have seen huge variances in the past with speed readings from day to day with the same boat. The last couple of years the selectivity has been turned off and the GPS's are now accurate at all times.

In my 1995 23 Nova I went from a 7.4/330 to a ZZ502/502hp. Gps speed went from 56-57 to about 68, with a Mirage 25P. 11-12 mph increase = about 15hp per each mph. That's just my boat, every hull will be different.

I had Throttle Up lab my 25P and gained 2 mph. So, the humble little Nova will do 70 gps on just about any day. Boat weighs 4000 lbs.

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Per Tomcat ---

Your top speed now suggests about 450 HP, but think about when the before speed was measured versus now. Changes in temperature and humidity have a significant effect. The prediction of 78 might have been a bit optimistic, but don't kill the engine builder yet, some investigation and more tuning and testing may be required.
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Read what Tomcat says about "temperature and humidity"....

On a hot humid day 69-70 is about my top speed. Late last summer on a day around 70-75* and relatively low humidity I saw 73+ on gps. That's a 3-4 mph difference!

If your bud has his prop labbed, he may see another 2 mph. That's a pretty common gain for a labbed prop. Then suggest that he run the boat on a cooler less humid day. He may see another 3-4 mph. Add the gains up and he just might see another 5 mph.

And as Wobble said, dyno is worth the $$. If he can't do a dyno, be sure it's tuned/tweeked properly.

All in all, I think the builder was too optimistic on the top speed. But it doesn't seem like your bud got hosed.

Good luck

3300lbs.
and no stepped hull............7'-6" beam, no pad.

He did tell him that it ran 62mph. And he did run the snot out of it everytime we went out.

But the boat ran 72mph 8yrs. ago because i got a new GPS and was trying & crying at the speeds compared 2 the dash speedo's. That was a big let down 4 all of us! hahaha

How many more things (weight) is he carrying around in the boat now as compared to 8 years ago?

As many have said heat & Humdity play a big roll. Here in AZ we don't have alot of humid weather but we do see the heat (it's a dry heat ). Spring and fall when the temps range 85-95 during the day my boat will see around 68 MPH running 5100 RPM, last weekend we were out temp was 116 boat wouldn't move could only get 62 MPH at 4700 RPM. Losing 6 MPH due to the heat, try some testing early in the morning when its cooler and humidity hasn't set in yet.

Sounds about right to me...


Jbravo.. You run the saginaw river/ bay? Thats where I do most of my boating. I run a 18' powerplay with a stock small block

id have to say that i would think the boat should run alittle faster too.
there was a 25 aggressr locally that ran a consistant (not accounting temp and humidity) 73-74. you could count on the boat running 72 all the time!!

talk to sunsation to see what they would expect with stock HP in the boat. then do the adjustments as needed.

im in the same "boat", my checkmate 242 ran 58 with a 330/454, now with a 550hp (dyno numbers) motor in the boat it JUST runs 70-71mph-gps.
i understand that with my exhaust and drive im probably only 500-510 at the prop.......but ive heard that the boat with a factory 502 it runs 68, an hp500 runs 72-73....knowing that and the 15hp/mph rule i should be 73-75. IM NOT!!! but im still tuning as Kidnova said.

I guess after reading JBravo, I am confused. You said the boat ran 72mph 8 years ago and now just before the new motor it only would run 62mph. My math is telling me thats a 10mph decrease before the new motor! What happened to the boat or motor that it slowed that much?, thats a lot! Sounds like there is some missing infor or you are not comparing apples to apples. This is why it is so important when a boater is trying to make speed improvements to a boat that you start with real up to date verified data and then make like comparisons as Tom's post talked about to really get the "black art" of fine tuning a hull, motor, drive and prop combination to its best result. Some boats have bottom and x-height issues that will drastically limit gains as well as other considerations such as CG, prop efficiency. You really can't compare different boats with different hull shapes and forms closely. An actual 525HP (real) out of a good 502 motor is not a big unrealistic number for any decent engine builder to achieve, so if your buddy is smart he won't throw the baby out with the bath water, but get busy fine tuning the whole package. If it ends up an engine builder problem, then deal with that accordingly. If it were easy, anybody could do it!

Be Patient! Good Luck,

Ray @ Raylar