Props - different for 4/2 stroke?

[Tim M]

I have a Ribcraft 4.8 with a 2-stroke Mariner 60 on the back. The boat is fitted with an A-frame and twin jockey seats.

The uni rib is exactly the same, only has a Mariner 4-stroke not a 2-stroke on the back. Jimbo has got 5 knots more with the uni rib than mine, which to me doesnt make sence because the 4-stroke weighs a lot more than my 2-stroke. I intend to raise the engine a couple of holes to see what happens; hopefully this will make a difference. My main area of concern however is the prop. I don't know what size my one is but will check. (I don't have a rev counter fitted yet but I wouln't say the engine sounds like its over-reving)

My question is: are boats comparable to one another when one has a 2-stroke and one has a 4-stroke in terms of prop size. Ie if I buy the same prop as the uni rib will I get a similar (probably slightly faster) speed?

Edit: the prop is 11 x 12

[ADS]

Tim, would I be right in saying the Uni RIB engine is a 'big foot', if this is the case im sure they have an uprated gearbox/ high thrust prop or something there is a difference, so I wouldn't go out and buy the same prop.
Get a revcounter and see what revs its pulling 12" sounds abit small on your application, I had a 13" on my 4m/ 40HP and that was very underpropped. You could take a revcounter off another of your fleet and just mount it for a few test runs.

[Tim M]

Arh yes I had forgotten the uni one is a bigfoot. I'll borrow the rev counter off Neptune and see what it says

Thanks Alex

[Tim M]

I phoned Ribcraft a moment ago just to see what they thought. The guy I spoke too said that the a good top speed for a 4.8 with 1 up was 35 knots which is the best he's got with a 60 E-tec (my best is 33, 2 up). I'm going to raise the engine a hole and see what happens, although I am a bit more re-assured now about my speed. He did say he reconed I could go up a pitch or two on the prop so I might get steel developments to bend my prop a bit and see if that gives me any more.

[Nos4r2]

I've got a 10.5x13" ally prop that'll fit yours-bring spanners and you can try it out on Sunday-or if you want it I've got a well shagged 14"pitch you can have. If you get that repaired/repitched then you'll have a spare too...

[Tim M]

Nos that would be great

[Jimbo]

Originally Posted by Tim M

Jimbo has got 5 knots more with the uni rib than mine, because he floats across the water with the grace and style of a sliver swan, as befits his skillful status...

Why, cheers Tim

[Tim M]

Hmm. Tell me more about when you got 37 knots out of it. Had you had a few cans of beans for breakfast or were you being pulled along by the solent dolpin?

[Jimbo]

Originally Posted by Tim M

Hmm. Tell me more about when you got 37 knots out of it. Had you had a few cans of beans for breakfast or were you being pulled along by the solent dolpin?

No no, it was quite simple really. I used a JATO unit.