Engine Height, Prop Slip, Prop Pitch

[copboat]

Hello All,

Got some hours on our twin 115hp 22' Nautica and I wanted to share some results and ask a few questions

OBSERVATIONS:
1) Prop slip with 3 blade stainless 17p is near or at ZERO, crazy
2) We have 25" shaft lifted to middle hole and then lifted again to 4th hole with no change in top speed, maybe slight reduction (1mph). With it mounted on 4th hole the spray between outboards got worse and we can slightly vent the props during tight radius turns on plane at 30mph
3) There is a fair amount of spray between the twins but when observing the outside edges we can see cavitation plate and looks about right height when on plane
4) Getting about 2mpg with her at 3/4 throttle

QUESTIONS:
1) Does anyone else have low prop slip with these RIBS? I didnt expect this but I guess it makes sense since there are 6 blades in the water with a 2200lb (gas/gear/ppl incl) boat. Dont get me wrong, this is a good thing
2) Given the cavitation on tight turns and possible 1mph reduction in speed we are going back to middle bracket hole, any other suggestions?
3) The spray between the motors is pretty excessive, is this a characteristic of close mounted twins?
4) We are going to up the pitch to 23p prop, should net ~55mph. Thats as much as we are going to get before drive ratio changes which probably isnt worth it. Whats a safe'ish 22' rubber ducky top speed? When do we have to worry about hull or tube dynamics?

Attached are some photos, after riding this thing through some 4's I'm not in a hurry to get back to my Formula 25 at all

Regarding the Mercury 115 2+2's, after rebuilding 8 carbs, ripping out the accelerator pumps and rebuilding the fuel pumps...quite fond of these simple 2 strokes. They idle well and really really like WOT. More quiet than I expected while at idle AND on plane. The downside is that there is a "high idle" vibration that is low frequency and really lights the center console canape structure up. But we just dont run it there...This outboard is kind of bashed in marine circles and they can go cheap. After running these for 300mi I wouldn't be afraid to recommend them.

[jwalker]

Me, I'd put them down 2 holes. Better grip on the water in rough conditions, better grip in turns, more bow lift...All plusses in my book. Get the engines down and the props fully loaded, test for max revs then decide whether an increase in pitch is required.

[Xk59D]

If it only cavitates on tight turns while trimmed up and the cav plate is just surface skimming while underway I wouldn't change a thing, just trim in when doing tight turns. Exactly how my boat is setup. I purposely have mine this way as going too deep in my case resulted in bad lean to port when underway, raising engine fixed it, everything is a compromise with setup.

On my rib I get 1% slip at full whack, I suspect the 20 degree Suzuki prop isn't actually a true 20 degree hence it being that low.

[FoxZulu]

How do you measure the amount of slip you are getting?

[Xk59D]

Easiest way is via mercury prop calc on their website.

[FoxZulu]

Thanks Xk59D. I'll take a look.