Engine trim problems

[Captain cod]

Thanks for the reply guys. I feel as thought I'm just going to have to spend a bit of time experimenting with different trim settings and pressures. Although I feel the tubes are at the correct pressure. The keel could take a few more psi I think. I'm not too sure which way to go with the new prop though. The one that came with my motor was a 16p but I never used it because it had a crack. And the one I've been using was a 13p but I don't know what the rpm were at wot

[Captain cod]

I feel you pain 9-R it's so frustrating. Knowing somthing isn't right. But knot know exactly what the problem.is when there are so many things that can cause them. My problems dont seem to as severe as the ones you had. I feel as though i just need to be a bit more patient and spend a bit of time experementing with different settings and keel presures.

I love my sib even though I break my back each time I set it up. I just wish I had the time to get out on her more often.

[Captain cod]

So I'm currently on holiday at pentewan sands in st austel Cornwall. I've managed to get out on the boat a couple of times. And when the tubes and keel are inflated to the recommended 3psi. The underside is like a drum. so didn't see the need to inflate any more.

Anyway when I go from idle to wot I get on a plane no problem and managed a top speed of 25mph. But same issues as before. If I'm only doing 1/4 throttle and speed up gradually the bow seem to push into the water. Not so much of an issue as if I want to go wot I just back off on the throttle first.

The other question I have is about the pitch size. At the moment I have a prop with a 16 pitch and the tachometer I have fitted is giving a reading of 4300 rpm or 8600 rpm depending on which setting it is on. I'm presuming it's the lower reading and the other is for 4stroke engines. So if it is 4300 rpm And my engine should be doing 5500 rpm at wot. Am I right in saying I should go dow to about an 11 pitch prop ? To increase the revs at wot to the recommended amount

[9-R]

Wow, 16p seems like a lot! I use a 17p on my 90hp.

I've used the general rule of 200 rpms per every pitch but not in that big of a leap so I'm not sure. In smaller jumps it's been accurate. Maybe can correlate, [thinking/typing out loud] - my 20hp w/ max of 6k rpm did it's best at 10p by myself and 9p with my family (around 5800 rpm). My 90hp w/ max of 6k rpm is awesome with it's 4-blade 17p (it did have a 20p and was sluggish. New prop increased our top speed by 8 mph). Add'l blade causes you to lose 100 rpm. I know difference in diameter makes a big difference but that's hard to change so keeping it out of consideration. I think the 11p may be slightly too low? Maybe 12?

25mph is fast in a SIB but there are guys doing 50mph in a catamaran style SIB with a 50hp so something does seem off. I was able to get a max of 21 mph with my 20hp 4s, 9p, 460 and my family of 5. Avg was 18mph.

[PeterM]

Firstly . Pentewan is a great place to boat - I'm envious ( but todays weather reminds me of when I was last there ! Wet Wet Wet * Windy ! ) …


My best guess is... you are the limit sizewise of SIB ( even the RNLI D class at 5M & 40hp is on the limits of what SIB can do - big & heavy ) ……….. and I suspect you are just suffering from this in that until you can get clean air under the bow its sucking down on to the water ( which can be a benefit if you happen to be in a D class and are in crappy weather and are rescuing someone !) .


If you go to full throttle straight away the bow lift means when you combine the increasing speed with the bow coming up you quickly end up running the boat with only the rear 1/3 ish in contact with the water - ideal !

If you cruise along the bow will sit on the water and even being drum tight will have enough give to suck itself down onto the water - and I just don't think there is a way to avoid this happening . You could try more trim out / up , but even then overcoming the change from displacement to planning has to happen somehow and at some time - going WOT lets this happen quickly and with the bow clear of the water , trim up may allow it to happen at a lower speed in a more comfortable way …….


Enjoy Pentwean - I'm hoping to get back mid September ! Has Jim the tractor driver smiled yet ?

[Captain cod]

Thanks for the replies guys.

9-R I think I'll have a look around for a couple of cheap decent second hand props with different pitches and have a play about with them. Once ive found one that works best i think ill get a nice stainless one. Would you recommend a 4 blade over a 3 blade??.

Peter M
Unfortunately I think you have it spot on mate about the max limits and weights. And I think its just somthing I'm going to have to live with.

But to be honest I'm a bit tired and fed up with my sib at the moment. It's just getting too much work to set up and use. And pulling my back all the time. I'm either thinking of getting a honwave t38 with a 20hp. Or dare i say it. after seeing some of the nice shiny white speedboats at pentewan. I could see myself in a nice fletcher arrowsport. But if have to convince my Mrs to let me keep it on the drive.

Jim did manage a bit of a giggle the other day only because he was laughing at me because I only caught 2 small mackerel. That were so tiny I had to put back in

[9-R]

I don't think you need a 4-blade especially with the size and weight of your boat. I got a 4-blade to help with pulling a large tube for the kids and wakeboarding/waterskiing.

[PeterM]

Its a slippery slope to go from small SIB , to big SIB to a small RIB . It is possible to have a more comfort on a harder hulled boat (and keep or get a little SIB for fun anyway). If you are getting Jim to launch your big SIB you may as well got to a small RIB - it will be quicker and comfier and more efficient than a SIB - but of course you loose the flexibility of storage and packing it away over winter etc and do need to think about storage.

I've run the little RIB (4m & 50hp) and little SIB (2.9 & 10hp) and have huge fun, but different fun in both TBH - launching either at Pentewan is either expensive £25 for the tractor or nearly killed me dragging the SIB on launch wheels through the sand


( oh and ajetski for being a complete speed freak at 50+ knts )