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Old 17 May 2015, 12:42   #11
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What they all said - they build 'em bigger over the pond (people that is - 21 stone?! ).

I don't understand the contradiction in your OP too: "Though, the engine still isn't giving enough WOT power. I get maximum 20mph with just me in the boat, 13mph with 3 people aboard.

The boat will just NOT plane on its own..."

I think given the loading (extreme for this size engine) and the fact that just one of the people alone you mention is 21 stone (again ) the performance is surprisingly good.

Small boats and low power engines are incredibly sensitive to loading and distribution, it takes only a slight imbalance to make all the difference. The solution is simple for your set up though - less weight or more power.

And as someone pointed out - it won't help much but maybe a tadge - it looks like you have too much leg in the water. Have a search for Locozodiac's tips on engine height.

Re' the engine cutting out - let it warm up a little longer and look at upping the idle speed a tadge either on the carb or keep the idle knob turned up until you are underway a short while.
Maybe my speed expectations were off a bit, I was expecting 25ish miles per hours alone, and 18-20 with passengers.

Speaking of contradiction, there is a strange contradictory story going on with me putting 15hp on that boat. You fellows are telling me I should be expecting less performance than I'm seeking with 15 horses, but many of the folk I've approached IRL tell me I should be screaming with 15hp on it. Truthfully, I'm not experienced enough to what to expect myself, or what story exactly to believe. Right now I'm just trouble shooting, figuring out all of the little (or big) things I can do to get this boat functioning properly. Things seem to operate *ok* when it's on plane, though I just have a really hard time getting it to plane even with just me on board and maybe 2 gallons of fuel left (which seems ridiculous to me, but as a noob what do i know?). If 15hp really is sort of underpowered for that craft, maybe i really do need a high-thrust prop instead of the stock pitch...
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Old 17 May 2015, 12:45   #12
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The engine itself is verifiably okay. It's had 3 different Mercury authorized shops inspect it. I took the cowl off, looked inside the carb intake and it opens up fully. The problem probably
lies with the type of prop I'm using, and perhaps how weight is being distributed. My boat has a couple attachment points on the inside of the bow (just like the tow line hooks on the outside), my guess is those are how you secure a bow fuel tank? I didn't see any strap loops on the Moeller bow tank, though.

I can try and get a picture of the engine on the transom, but the shaft length is correct.

He was indeed a heavy guy, a little more than 130lbs more than I and effectively counts as 2 people. Though, he was sitting on the front seat along with my other mate. It was easier to get going with them up front but we were still just chugging along at 13mph (I would think we could maybe do 20mph, or is that unrealistic with, effectively, 4 people aboard?). That's how we were set up during the video I linked prior.

To answer the question as to whether it was in shallow drive mode, it was not. I had it in the lowest trim for the vast majority of the time we were operating. I tried the two slightly higher trim settings, but it didn't help so I just left it back in the lowest trim.

I can try a bow fuel tank, but moving *everything* and everyone up front to the bow to where I myself need a tiller extension is nothing short of ridiculous, and to the point where I'd just sell the boat. Nearly every video I've seen of someone operating an inflatable, they're running a stern fuel tank with all their gear towards the rear, yet they're still getting up on plane quickly and running along much faster (even with just 9.9hp motors).

The only things I can think of that might be wrong, except for the fuel tank moved forward (which I'm skeptical of, because when I was attempting to operate solo, I was on low fuel) is that I'm over-propped and need to go to a 4-blade lower pitch prop, or that my Bravo pump is reading wrong and not getting everything inflated to 3.5psi.
Try a 7.5pitch prop or similar.you can also be to front heavy .At what speed does it get on and off the plane.
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Old 17 May 2015, 12:57   #13
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Think you just aren't getting it... You have a lot of payload on that boat. What's its capacity? In European money assuming your third crew is an average 70kg you are carrying 375kg of people, plus 20kg of engine and at least that in fuel. Someone mentioned spare fuel, battery anchor. Guess the transom wheels don't weigh that much but they are by nature at extreme back... That's gotta get you close to 500kg

So in short you are always going to struggle with getting on the plane and will need to distribute weight according including while underway.
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Old 17 May 2015, 13:01   #14
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The only things I can think of that might be wrong, except for the fuel tank moved forward (which I'm skeptical of, because when I was attempting to operate solo, I was on low fuel) is that I'm over-propped and need to go to a 4-blade lower pitch prop, or that my Bravo pump is reading wrong and not getting everything inflated to 3.5psi.
Read the replies! A prop won't solve your problem - losing the equivalent of a newborn baby elephant will.

Re' set up again; generally the lowest trim position will not be best, most run 2nd or occasionally 3rd out (of five say). Get a Tiny Tach (or 10 ebay equivalent) and do some test runs to check trim. Leg length is just one aspect - due to variances in transom height and boat set up you may well need a wooden raiser under the engine to bring it up 1 - 2 inches.
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Old 17 May 2015, 13:55   #15
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Second Time Out, More Problems

Agree with max on this, suggest moving weight around, and ensure you are running the engine at the correct depth, I had a zodiac with a 9.9 and a 20 hp on the back both pushed boat along real nice on plane in no time.

It's all about weight position and trim !!
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Old 17 May 2015, 14:19   #16
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>>>there is a strange contradictory story going on with me putting 15hp on that boat. You fellows are telling me I should be expecting less performance than I'm seeking with 15 horses, but many of the folk I've approached IRL tell me I should be screaming with 15hp on it.

To pick up on this...

If you put my teen daughters at about 110lbs each into a 3.5m SIB with 15hp, one on the tiller and the other nearer the bow, with a 1/2 full 10l fuel tank, transom wheels off and no extra kit then... yes... it would be "screaming along". It would jump onto the plane with perfect balance and probably max out the engine revs on a standard prop for (I'm guessing) about 22mph.

The boat will feel lively to the point of even being a bit twitchy at speed and you'd be getting the best from it.

If you are about 140-160lbs then with just you and minimal fuel/kit the performance would be similar with a tiller extension to get you amidships.

Then every single pound you add to that setup will reduce performance to the point you have reached with a load that is more suited to displacement speeds and now the boat is so resistant to getting on the plane it is looking for every excuse not to... or to fall off at the slightest excuse.

If you get out in a bit of a lumpy sea a lightly loaded SIB with the ability to pop on and off the plane in seconds with a twist of the throttle is great fun and quite safe. A sluggish heavily loaded SIB is unrewarding, can ship loads of water and become less than safe.

For the loads you want to carry a 4m with 20-30hp would be far more suitable.
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Old 17 May 2015, 14:49   #17
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Just a thought - do you realise that a tiller extension is there to help you get on the plane? You don't need to stay forward. The technique is slide forward a couple of feet and get on the plane, then you can return to the stern. It helps if you move heavy stuff forward too. It's boating 101. Try my suggestion and see how it goes - you might save yourself a lot of hassle and money.

300lb guy would make a good shore marshall though
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Old 17 May 2015, 14:54   #18
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300lb guy would make a good shore marshall though
Eh, what are you saying about your last shore marshall?
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Old 17 May 2015, 14:58   #19
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There is a general rule of thumb....it's not gospel but a good guide that says you need about 50hp per ton to get planing performance. You're a bit below that so if you get there then it's a result.
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Old 17 May 2015, 15:11   #20
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Eh, what are you saying about your last shore marshall?
A good shore marshal should be solid and dependable
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