3.3 SIB / Speed

[Morse]

Hello looking for some help and thoughts on the following.

I have a 3.3m Excel volante with a 15hp Yamaha 15hp 2 stroke.

With 2 persons and when the planets rarely align I can get about 20 / 22 mph.

The majority of the time 15 / 16 MPH seems to be about the norm.

With 1 person I was getting 15 mph and 16 at a push playing about with the trim settings. I could get no where near 20 MPH.

The only issue is that F15 is designed for a 440 transom. The excel is 385. I have tried a block of wood (32mm) and that brough the speed up a few MPH averaging about 18 / 19 MPH.

The issue with putting in a bigger block is that it leaves very little of the transom for the engine to grip.

I have played about with the 4 trim settings on the engine. Further out causes cavitation and is unusable. The others don't make much of a difference.

My last observation was that WOT makes a lot of noise and nothing else. I can put it back to about half throttle and get the same speed - about 15MPH.

Am I expecting too much for the type of boats, should I be able to get a little more speed? I understand there are lots of variables and the above figure come from playing about on flat calm conditions in the Clyde.

Thanks,

Gordon

[Locozodiac]

Sit the motor flat on transom, with 2 up sit a boater up front, trim the motor to 90 deg (fully perpendicular) when at full plane on flat water pull your head side of motor and check at which lower leg's height is the hardened water passing by. Is it passing under the small upper plate or over the larger AV plate. Ideal is to pass under the small upper plate, that's where the motor achieves its max prop thrust in all water conditions. Try to sit the motor at a certain transom height to achieve that condition. Report your findings...

Happy Boating

[Fenlander]

Hi Gordon and welcome to the forum.

I've run several different 15hp motors including a Yamaha like yours on several SIBs from 3.4m-3.8m so have a good idea what to expect.

I tend to think in Kts but converting my experience to MPH I would be happy 1 or 2 up with your 22mph but not with the 15mph you've seen 1 up. I can only guess that your SIB is poorly trimmed 1 up with all the weight to the rear... can you move the tank and any other kit further forward plus worth considering a tiller extension?

Also unless a very strong headwind or tide I find the maximum speed of my setup quite consistent... if it will make 20mph that hardly varies from one moment or trip to another.

I see you report an improvement in speed with a 32mm block and say any more would mean you were not fully clamped on. In truth taking the 385-440 difference the 32mm raise is within 20mm of correct and I have never found a 20mm height change to make a significant different with these portable motors on a SIB which is after all a floppy and imperfect shape to push over the water... it's not like tuning a GRP racing craft.

What's not been mentioned is your prop pitch... you really need a small cheap tach to find out if the OB is making its maximum revs at full speed. I wonder what your prop pitch is at the moment... it should be marked on it somewhere you can see?

Finally if you did want to raise the transom to match the 440 have a look at the work I did to mine. This was purely to stop water splash not related to speed but is an example of a more involved job than just dropping a small block on.

https://www.rib.net/forum/f50/aerote...e-71528-2.html

Posts #69 to #78

Finally I see you have the air floor Excel. Are you sure the tubes and floor in particular are fully up to pressure with a gauge you trust... and is a feeling of prop slip part of the reason when the speed is not what you'd expected?

David

[Wazzajnr]

For reference, I've just been on ullswater today and we were cruising at around 11-12mph (I know, just over the limit but slipped off plane at about 11 so was a pain)
I did give it a quick spurt to wot and got 19mph from it.
Ours is 3.2m ally floor with two adults and one 9yo on board. Motor is a circa 1980 evinrude 9.9hp.
Very splashy at the back but I think my motor is way too low in the water.
You should be easily able to get 20mph with a 15hp on the back with a sensible load on board.

[Morse]

Hi David,

Thanks for the reply.

I might have been getting a little mixed up on the science of trying to get up on the plane.

I was trying to put the OB trim to its furthest out setting to bring the nose up. It cavitates really badly so that rules that one it either with 1 or 2 people.

With 1 or 2 people we have been sitting at the back to bring the nose up and probably because I don't have a tiller extension this is the only real option or at least it with just me.

With 2 people I have moved one person forward to bring the nose down and then as speed increases move back again.

Someone suggested to me that with the likes of the tiller extension I should be much further forward and let the speed and the bow do the work getting on the plane. This differing somewhat from me just using my weight at the back to bring the front up like a see saw would.

Load wise the 24l fuel tank sits at the front but other than a peli case, jacket etc that's about it.

Cheers,

Gordon

[Wazzajnr]

The power of the motor will be plenty to bring the nose up. Most sibs have an angled transom so I'd be inclined to get weight as forwards as possible with the motor trimmed right in and see what that brings.
We have boy and wifey on front bench and I sit on the tube as far as I can get and still reach the tiller.
It will get on plane like that but if I lean forwards, it climbs out quicker. Once on plane I can sit back a bit.
Also worth checking your motor height as the deeper it is in the water, the more bow rise and drag, both of which will hold you back.

[Steve509926]

Hi Gordon and welcome.

I have a SD360 with a 15hp 4s. Perfectly trimmed, one up, correct prop 6100rpm @ wot, redesigned keelboard, and I can manage 17.5 knots just over 20 mph.

Lots of things to take into consideration.

Volante excels ain't no speed machines.
Excels appear to sit stern high in the water so you may find having your AV plate slightly below the keel is the best position. Most Excel owners that I know do not have any packers and sit their OB's directly on the transom.
I've found that pressure is critical, really critical! Inflate everything to the correct pressure, then after being in the water for 15 mins reinflate to the correct pressure, especially the keel and the airdeck.

Lastly, but most important. Don't let that lack of 2mph spoil the enjoyment of a brilliant, comfortable, stable sib.

[Fenlander]

>>>I might have been getting a little mixed up on the science of trying to get up on the plane. I was trying to put the OB trim to its furthest out setting to bring the nose up. It cavitates really badly so that rules that one it either with 1 or 2 people. With 1 or 2 people we have been sitting at the back to bring the nos.

Yes you have been getting it a bit wrong. Indeed the bow will come up when you are going onto the plane but this is actually a brief undesirable if inevitable attitude of the boat for a few seconds. Actually you want weight forward to minimise bow rise so getting on the plane is achieved quickly.

[Morse]

It is the case that when I start to move off I have been putting weight forward other the bow is up in the air. This gets the bow down and moving up to 15mph fairly quickly. That's where I max out however by this point I have been then moving back to take the weight off the front thinking this gets the bow up a little and reducing drag. Am I wrong on this point?

I will try today getting further forward even when up to speed or up to what currently seems to be the top speed.

[Fenlander]

Ahh OK I misunderstood... yes weight forward to get on the plane is good if you are getting excessive bow rise. But once on the plane it is rare to need to get to weight to the transom.... just have the boat evenly balanced.

Yes in theory a fast boat on the plane will run on the rearmost part of the hull but with a SIB the usual way is quite an even loading when on the plane.

I usually sit on the tube a comfortable but longish arm reach to the tiller. Mrs F sits on the central seat... the fuel tank and heavy anchor/kit bag under her seat... plus the dog and any day out kit at the bow. Our SIB goes onto the plane with a slightly more level attitude than most so we usually don't move weight about.

[Morse]

Thanks David,

So today's question will be if moving forward picks the speed up a little.

I wasn't expecting to be getting 40mph but I was surprised that 16 was maximimim speed although as I say that's me at the very back and fuel can and little else at the front.

I might be able to knock up a tiller extension but failing that I'll see what I can do.

Will let you know

Gordon

[909]

Holeshot:

Zodiac 340 H2pActiv8
1976 Evinrude 15hp
1 Person 200lbs
25L Fuel
15lb Anchor
20lbs dry bag

[Fenlander]

km/h in Canada I guess... so the max 38 is 20.5kts which is on the good side of the speeds I've seen lightly loaded and same hp OB and SIB length.

[Morse]

I had a bit of a breakthrough.

I moved the seat forward as suggested and went out into the flat calm. As expected I got up to 15 MPH and just lots of additional noise at WOT.

Reducing the throttle I was resigned to no real difference but I must have made some minute adjustment as suddenly the engine noise / load / strain changed, the spay either side of the bow stopped and it was an instant 5 mph up to 20 MPH throwing me back slightly in the process.

It seems I haven't been on the plane but only having used the boat 4 times I assumed the 15 MPH was on the plane. It wasn't until this happened yesterday I could see the difference. A kind of Eureka moment you could say.

At the point I brought the throttle back to half or slightly below and it kept the 20 MPH speed. WOT throttle maybe got up to 21.5 MPH at a push.

I tried a few times and although I'm not sure exactly how to get it on the plane I'm going more or less up to WOT and working back slightly until it gets up. I don't have any instruments beyond speed so I guess it will just be practice.

Sitting forward gives an extra 1 MPH. I assume 20 / 21 / 22 ish MPH is acceptable for a 3.3m SIB and 15hp 2 stoke. That was with one person 24l fuel and a few odds and ends.

The increase from 15 to 20 MPH actually makes me feel as if I can venture further afield.

Thanks everyone for your help

[Steve509926]

Happy days

[Dydigital]

Please come back with some more info as this will be very useful to me and others.

Sounds like the engine is bogging down and come back down the power is giving it time to climb. It's a characteristic you see more with big sibs with small engines.

Originally Posted by Morse

I had a bit of a breakthrough.

I moved the seat forward as suggested and went out into the flat calm. As expected I got up to 15 MPH and just lots of additional noise at WOT.

Reducing the throttle I was resigned to no real difference but I must have made some minute adjustment as suddenly the engine noise / load / strain changed, the spay either side of the bow stopped and it was an instant 5 mph up to 20 MPH throwing me back slightly in the process.

It seems I haven't been on the plane but only having used the boat 4 times I assumed the 15 MPH was on the plane. It wasn't until this happened yesterday I could see the difference. A kind of Eureka moment you could say.

At the point I brought the throttle back to half or slightly below and it kept the 20 MPH speed. WOT throttle maybe got up to 21.5 MPH at a push.

I tried a few times and although I'm not sure exactly how to get it on the plane I'm going more or less up to WOT and working back slightly until it gets up. I don't have any instruments beyond speed so I guess it will just be practice.

Sitting forward gives an extra 1 MPH. I assume 20 / 21 / 22 ish MPH is acceptable for a 3.3m SIB and 15hp 2 stoke. That was with one person 24l fuel and a few odds and ends.

The increase from 15 to 20 MPH actually makes me feel as if I can venture further afield.

Thanks everyone for your help