small speed boat big engine what prop?

[inohv8]

Hi.<br />I have a 10 foot Peterorough fun boat.. I re-built a 50 short shaft for it ( I know, oversized), but I will be driving it, not my son..<br />Anyway, the engine is a 50horse with a prop size of 11.75 by 17, I know my engine will over rev, so a tach is going right in my face. If I go for a cupped prop at 19 pitch, would I be safer on the rpm? I once had a 40 on there and obviously went like stink, glad it is a stable hull..<br />Any thoughts other than installing a smaller engine? on what I might gain or lose by going cupped and higher pitch?<br />tks.

 

[inohv8]

Re: small speed boat big engine what prop?

If this helps, I should add more info..<br />the boat is a two seater closed bow, the engine is a Johnson 1988 or 89 twin cyl. not water tested as of yet, only barrel run. have to wait till the ice is of the lake.

 

[KCLOST]

Re: small speed boat big engine what prop?

No way of telling....<br /><br />But your a brave soul for trying it out and testing.... Don't want to chine walk with that kind of power... In addition to over reving the engine as a big worry, speed and possible chinewalk is even worse...<br /><br />Good luck and be careful... Keep in mind... More pitch equals less rpms and more speed up to a point.. And it's hard to tell how much extra pitch you will need to get to the optimum rpm level at WOT and at the same time prevent yourself from taking off like an airplane or chining quickly!

 

[inohv8]

Re: small speed boat big engine what prop?

I appreciate the feedback. The vessel is rated for 30 hp, however with myself and my son or about 250 lbs total payload (not counting fuel and battery), the 28 horse I have on it now is a real dog, and hard to get up on plane, the 40 I had before (merc) was real sweet up and out in nothing flat, very stable but lost the ignition and none available, so off it came. Since it's a two seater, can't go tubing so it is basically a beach hopping boat, I never go far with it (to small for the swells). Thanks.

 

[Kenny Bush]

Re: small speed boat big engine what prop?

Boat is short and tail heavy, so it will always be slow out of the hole. You could prop down to 15 and get out quicker, but you wont like the top speed. If you are running a 17 now, you may be in the zone. Just wait and test it, then post with what direction you want to go. One thing I would recommend is a motor Foil (Stingray, Doelfin, ect..)mounted on the cavitation plate. This will help plane out faster, and give you a little tail drag (which you will need at high speed).

 

[WillyBWright]

Re: small speed boat big engine what prop?

Since you're overpowered, all I'm going to interject is that the use of trim tabs could add some stability. I think you'd do better with those than a whale tail. The tail lifts at the center and could adversely affect stability, and constantly changing trim and steering angles kinda throw those forces in odd directions sometimes. Tabs are out on the sides for added stability and they aren't changed every time you steer it or hit the Trim button.

 

[inohv8]

Re: small speed boat big engine what prop?

Boat is short? yes, tail heavy? No, much lighter than the 28 horse (1963), I always use the doelfin.. the 28 had trouble getting out of the hole but the old 40 was like a jack rabbit.. I a just don't want to over-rev, so I have to prop it with the correct pitch, so as I understand it, I can go for a cupped prop and higher pitch to lug the engine down, but being a 50 it should not lug please correct if wrong..<br />tks.<br />stan

 

[inohv8]

Re: small speed boat big engine what prop?

I just had a look on the transon, there actually are trim tabs at the bottom of the transom by the chines

 

[WillyBWright]

Re: small speed boat big engine what prop?

I'll also throw in that ventilating a steep prop can help with holeshot. But you didn't hear that from me.

 

[Paul Moir]

Re: small speed boat big engine what prop?

The 50hp will weigh an about an extra 60-100 lbs depending on options. If you can't get a vessel rated for 30hp on plane with your 28hp, I think your engine must have problems. I wouldn't go out in it with you unless you had re-enforced the transom.

 

[inohv8]

Re: small speed boat big engine what prop?

The 1963 28 horse evinrude wook all my stength to mount it on the transom, the 50 horse johnson (1988 or 89) took some hernia power but probably about 30 lbs lighter.

 

[Paul Moir]

Re: small speed boat big engine what prop?

Check if you want to, NADAguides.com list dry weights. The '88-89 50hp (2 cyl) is 186lbs min 218lbs max (w/ tilt&trim, etc), the '63 28hp comes in at 130 lbs. (www.old-omc.de).<br /><br />For what it's worth, my grandfather had a cedarstrip Peterborough back in the '60s (about 15' if my memory serves me alright) with a '62 Johnson 28hp on the back. They used to pull waterskiers around with it - big fat ones naturally (the skis! ). If your 10' is anything like that '15, you really need to beef up the transom.

 

[inohv8]

Re: small speed boat big engine what prop?

I'm surprised by the weight but I won't deny it.<br />I know I had the 40 merc on there and the transom was and still is super solid.I flew around doing donuts. There is a lot of support there as the transom is narrow (big enough to fit the engine, no room for a kicker. I made up a bracket to hang off the side( like a canoe) to hold a 3.5 to get me home (just in case). Tks for the info.. care is the utmost concern.. noted ...<br />stan