Originally Posted by Locozodiac
1-Way to begin with, newbies likes playing too much with trim settings on initial setups, which isn't the way to go in order to maintain best prop thrust.
2-How many mates were on board when tested ? Assume deck was well weight ballanced ?
3-Motors are water devices to push just anything that floats, inflatables are water gadgets to have fun with, not necesarily will perfect match between, in most cases will need a engine/transom state of the art height adequacy.
4-After re inflating sib to its fullest tube and deep V deck working pressure, check water flow again, if the AV plate is still exposed when sib is riding parallel to water level and engine at 90° you definitely need a transom height modification. Ideal is water flow skimming right under upper water defector plate. If you already are achieving aereation problems on straight watercourses imagine at close tight turns at speed.
It's useless buying another sib before any attempt is done to have actual one in opt operational fun state, you need to chop transom down at least 4 cm to lower engine for flow to pass closer to upper plate. Once there combo will perform much better. Test on flat calm no wind sea cond preferably.
PD : Chopping transom down does not mean the whole transom lenght, just engine's width middle portion. Just center well engine on middle transom and chop, make 2 plywood plates, screw engine's clamps tight to wooden plates momentarily.
If still not spot on dialed, chop 1 cm more, once dialed raise both original engine's plates to be even with chopped middle transom border. Engine will have a thicker border to sit and work much better.
Hi, thanks for the well considered response.
Q1, I read your set up guides re trim and height so I had some good info to start with.
Q2 we were 200kg of people with around 15kg anchor and chain and 12kg of fuel forward in the boat. Seats, boat hooks, oars, spares etc all rounded up to around 300kg maximum. Driver at the back and passenger in the middle on opposite tubes.
Q3 Looking over the transom under medium throttle, the flow looked fine until the throttle was increased - and the av plate broke the surface leading to the cavitation. This is with the AV plate already 2" lower than the bottom edge of the transom.
Q4 I agree and that's pretty much my conclusion too. The transom is already lower in the centre by design.
I could sink the prop still deeper by refitting the 5" longshaft conversion I removed to make it a short shaft, and building the transom up to be higher in the centre until I find the optimum height
Cut down the transom in 1cm stages until the optimum is reached.
My reluctance with the first option is that the motor currently fits across the car boot nicely and the rolled up boat fits in too. With an extra 5" it becomes more difficult to transport.
My reluctance with the second option is simply that its fairly invasive surgery and I assumed that there must be an engine / boat combo that already worked when Honda specified it without doing all the mods. I guess I'm thinking about resale value for the sib too.
If the 15hp 2 stroke is too light on such buoyant tubes and sponsons, it will be less expensive to swap the boat than buy a bigger heavier motor. Maybe I just need a buddy to sit further back with me to sink the transom more!
The input is much appreciated. Maybe more on the water experimentation is needed with some photos.