How fast should my boat go?

[Newbie@boats]

I have a 1983 Sea nymph 17FT with a Yamaha 50hp 4 stroke on the back, all the woods new in the boat so I know its not water logged and heavier than it should be....2 batteries, a trolling motor, and a 6 gallon gas tank....Now on gps the boat will do 32MPH with 2 guys and all our tackle.....RPM on the engine is 5200 which is WOT....Sound right? Could I change prop pitch to try and get a few more MPH out of it? Boat gets up on plane in seconds...

 

[Scott Danforth]

65mph on the trailer while tooling down the highway

on the water, that is about it for your motor with that boat. changing the prop pitch to a higher pitch prop would lower WOT RPM and lug the motor. you can try backing off on the pitch to bring your RPM up as the WOT for the F50 is 5000-6000 RPM may help a bit

 

[Newbie@boats]

[h=1]10 1/4 x 14 Looks like thats the stock pitch that came from the factory? You think going to like a 13 or a 15? Id like to get 35MPH out of it-don't ask me why.....im just crazy[/h]

 

[ajgraz]

I dunno if you're ever going to hit 35mph with two guys and gear and only 50hp, but as noted IF you are going to try another prop, you should go down an inch (maybe even two) in pitch to try to get full-on WOT rpms a little higher into that 5000-6000 range for that motor, if only to extend its life.

According to prop slip calcs, if you've got a 1.85 gear ratio (from web searching), a 14p prop, hitting 32mph at 5200rpm, that's 14% slip...which is not great, but not terrible, I'd want to see closer to 10-11% on a fairly lightweight planning hull. You might try moving the motor up a little on the transom (what folks here call the "x-dimension"), maybe 3/4" to 1.5", to try to reduce lower leg drag. This could both improve slip percentage and increase WOT rpms...just make sure you don't go so high you lose "bite" on the water (and thus, control)!

If it was me, I'd want it to hit ~5800-6000 rpm WOT with a light load, and at least 5500rpm WOT with the heaviest load you'd run in that boat. And if you can find a single prop that'll do both AND give the holeshot, "get-on-plane-ability," and control you want under both conditions, you win! Speed (mph) would be a secondary consideration IMO.

(Basically, if you want considerably more speed, you'll need more hp!)

EDIT: BTW, do you have power trim? Did you optimize it while underway to get the numbers reported above? If no power trim, have you played with (fixed) trim settings?

 

[Sea Rider]

Yams usually rrevs 5000-5500 wot rpm. If so, changing 14 pitch OEM delivered prop for a 13, boat will achieve a faster hole shot, but with slight top end speed loss. Will need to ask yourself if liking best hole shot or best top end speed as usually loaded. At 5200 wot rpm OB is revving at middle wot rpm range which is excellent.

Happy Boating

 

[Newbie@boats]

Yes I've played with the trim and if trimmed all the way down the engine will run 4800 at wot, if I trim it to its sweet spot I'll get to the 5200rpm.

The hole shot is excellent so I feel as if I don't need to try and improve that...I'd just like a little more top end speed boats rated for a 60hp I would imagine if I found a 60hp 2 stroke I'd get definitely a few more mph out of it but I just completely went thru this motor and got it running perfect.

So if I'm at a 14 now I should try a 15/16 to get a few more MPH/RPMs out of it? The prop is a little dinged up so this is a good excuse to buy a new one

 

[ajgraz]

I would not recommend going up in pitch.

 

[82rude]

Going up in hp will not give you a lot more maybe 35 to 37 maybe.I have a perfectly propped excellent running 2011 etec (60) on the back of my old starcraft ss 160 and 34 to 36 is absolute tops in perfect conditions.Now mind you thats with a load of me ,food,24 gallons normally of fuel and a extra 5 mixed and a 6hp johnson kicker sitting beside the 60.Normal top speed is 34 to 34.5

 

[Chinewalker]

So if I'm at a 14 now I should try a 15/16 to get a few more MPH/RPMs out of it? The prop is a little dinged up so this is a good excuse to buy a new one

NO. Go down in pitch. Akin to downshifting in a car. Going down in pitch brings RPM up. Try a 13.

 

[Sea Rider]

If going per 15-15 pitch props, you're going in the opposite rpm direction, rpm will drop drastically will end lugging badly the OB. There's a max top end speed combo can achieve, impossible to ask for more if OB is not changed for a higher HP-OB. You can always run combo much lighter to slight better top end speed.

Happy Boating

 

[b.gagnon]

I had a 1981 17' seanymph w/90hp merc 2stroke turning a 17p Stainless prop and it would hit 37-38 MPH....
I would say that you are getting great performance for a 50hp.

 

[David Young]

I agree. 32 mph on a 17 foot boat with a 50 horsepower motor, is all that should be expected

 

[fishin98]

I agree. 32 mph on a 17 foot boat with a 50 horsepower motor, is all that should be expected

I agree....your doing good for a 50, if you want a better hole shot dropping to a 13p is your answer, then again you will lose out on top end. My brother has a 16ft Sylvan back troller with a 50hp Mariner 3cyl ...gets 29 mph GPS indicated, with 2 large people and all the gear...