boat speed

[guido469]

Ive got a 16ft lund mod v and i cant get it over 30mph. I used to have a 25hp 1977 evinrude with a 9 1/2 x 11 prop and it ran 28.6 all day. Now ive changed to a 1985 50hp mercury with a 11 1/2 x 13 prop and it only runs 30.3. This is the third prop ive tried- 12x10, 11 1/2 x 14, and the 11 1/2 x 13. They all run right around the same speed, just different rpms. Was wondering if anyone had any explanations, cause im all out of things to try. I changed motors cause i needed more power when duck hunting, and wanted a lot more speed fishing. i just cant figure out why double the horse power at only 60lbs more only gave me only 2mph?

 

[BaileysBoat]

Re: boat speed

Is the motor at the correct height? Have you checked throttle plate opening at WOT?

 

[guido469]

Re: boat speed

yes height is correct. not sure where that plate is at, but the boat jumps out of the water and gets the r's fast. and it seams to run very good other than top speed is what it is.

 

[guido469]

Re: boat speed

cav plate is even with bottom. ive got elec trim so ive been able to run at optinum rpm. as for rpm the 10 pitch was around 61-6200-way to high. the 14 was like 52-5400. and the 13, which i stuck with is between 55and 5700. the speeds are very close. the 10 ran 29 and change and the 13 rans 30 and change. 14 was about the same. i stuck with the 13 cause it still gets me out of the hole good and gets the r's fast.

 

[reelfishin]

Re: boat speed

I've got a 1971 Evinrude 25hp on a 15.5' V hull by Grumman, the boat does 28.8 mph all day long, not much seems to affect the speed so long as the boat is balanced. I hung the same motor on several flat bottom boats and got nearly the exact same speed. It dropped to 28.5 mph on a 17' mod v. with no interior other than the factory diamond plate deck. I run the 9.5" x 11p prop that it came with.
I was also running a 15hp on the same boat, doing 23.5 mph with a Mariner 15hp on the same boat. My 14' Duranautic, with bench seats and a console does better, it runs about 26.6 mph with a 25 Merc, yet only hits 32.4 with a 40hp Merc, and I see the same sort of reaction to prop changes, I've tried various props, all run within a mile or two per hour of each other, and only RPM changes.
From what I found, the fine tuning to get more speed will be in the weight distribution and balance in the boat.

 

[snowman48047]

Re: boat speed

I've got a 20' Lund with a 50hp merc bigfoot (I duck hunt too) and my top speed with just me is only about 23-24 mph @ 5600 rpms. I'd say with your set up you've got to be close to top speed. I think your rpms should be around 5600.

 

[guido469]

Re: boat speed

i wonder if i get a jack plate to get the motor deeper will increase my speed.whats everyone think?