Outboard trim on small inflatables

[mikedefieslife]

How do you guys trim the outboard on your dinghy?

At the moment I've got a 2.60m air deck dinghy with a bit of a V, and a 6hp Yamaha 2 stroke. The engine is powerful enough to get the dinghy up on the plane with two adults onboard, and a plenty of shopping. The dinghy is rated up to 8hp (4 stroke)

On my own it will propel me to over 15 knts. Which feels like the end of the world. It's very skittish, small waves or undulations in the water will almost throw the user off or bounce them around hard.

The dinghy is very wide for its length. It's 160cm. The tubes are 42cm. I find this a problem as the tubes slap the surface of the water sending spray everywhere in any wind. I'd like to trim it with more of a bow up attitude, but that is proving difficult, especially with a transom that angles outwards.

At the moment I have the tilt/trim pin in the third position from furthest out. In this position the bow will raise at least 45 degrees just before getting onto the plane, where it flattens out. The problem is it flattens out too much.

In any other position, closer in or further out, I find the outboard raises itself out of the water and starts to ventilate.

Am I onto a loser with this or are there any tricks I can use to getting performing as I'd like?

[a64pilot]

Engine trim and center of gravity go hand in hand.
In other words sitting further back will raise the bow likely more than engine trim, but almost all of us need to sit further forward to get good boat trim, that is what all those extension handles are about.

Usually you can tell when an outboard is trimmed too high because the boat will porpoise, meaning the bow constantly raises and lowers even on smooth water, same if the CG is too far aft.

I’d assume your dinghy is either not fully inflated, especially the floor, or it’s just a normal condition of an air floor dinghy, I know the floors get a hump in them under way, but as I’ve never owned one, I can’t comment with experience.

[tkeithlu]

Aside from moving the CG forward as A64 suggests, you may have a rigidity problem, particularly in your inflated keel. Is the boat really tightly inflated? That skidding around does not sound healthy for carbon based life forms.

[mikedefieslife]

The air keel and the air floor that sits on top of it is tightly inflated. Without doing that then you don't get a V. In my opinion the V is too shallow compared to width and the bottom of the tubes, so it could just be how the dinghy is. I think its days are numbered anyhow.

With my old 2.5 (3.3hp) I remember when planing that the ventilation plate on that would also come out of the water too.

[tomfl]

Probably should provide the name of the inflatable.

[Shrew]

How long is the Dinghy? An 8'6" dinghy (260cm) with a 6 HP should not handle as tenderly as you describe. I have a friend with a 117cm dinghy and 15 HP. That gets a little squirrelly at WOT with a single person but is otherwise manageable.

Is this 6HP a 2stroke or 4 stroke??

[a64pilot]

Originally Posted by mikedefieslife

With my old 2.5 (3.3hp) I remember when planing that the ventilation plate on that would also come out of the water too.

When perfectly set the Anti ventilation plate should run just level with the top of the water. Little boats that’s hard to do and often you have to run a motor a little deeper to keep the prop from ventilating.

However that is up and down and not trim.
I wonder if jack plates are made for small motors?

[Shrew]

Originally Posted by Shrew

I have a friend with a 117cm dinghy and 15 HP. That gets a little squirrelly at WOT with a single person but is otherwise manageable.

CORRECTION: 9'9" (297cm) with a 15 HP

[mikedefieslife]

Originally Posted by tomfl

Probably should provide the name of the inflatable.

No one here will have one.

It's an Excel Volante 260
https://www.excel-inflatables.co.uk/...boat-sd260.php

The company is run by a British guy, that seemingly has them specced/made in China. Actually got an alright review a UK magazine a few years back against some decent competition, and some favourable discussion on some UK forums which is why I got it. https://www.mby.com/gear/the-ultimat...r-test-26057/4

Oddly enough, I've seen a few (for the same model) on my travels around the Med.

Originally Posted by Shrew

How long is the Dinghy? An 8'6" dinghy (260cm) with a 6 HP should not handle as tenderly as you describe. I have a friend with a 117cm dinghy and 15 HP. That gets a little squirrelly at WOT with a single person but is otherwise manageable.

Is this 6HP a 2stroke or 4 stroke??

Haha, I was actually reading an old thread the other day about someone with a 2.1m dinghy with a 25hp engine on it.

[txg]

Get a doel-fin:
https://www.davisinstruments.com/pro...zer-fin-black/


Helps not only a lot with getting up on plane but makes a big difference in planing stability when it gets a little choppy. On our 2.4m RIB with a 9.8 Tohatsu, it takes off about 2 knots of top speed (16kt instead of 18kt with 2 rather light persons onboard) but the gain in stability is absolutely worth it.



With only one person in our dinghy, we tend to sit completely in the back when on plane. It feels like most of the weight is lifted by the fin.

[Chuck Hawley]

OK, here some pointers based on sourcing and selling inflatables for about 30 years.

1. The floor pressure is critical, and the keel is less important. I think of the floor as being a given (11psi or so) and the keel as being a variable.
2. A short inflatable with any sort of floor is very squirrelly, and I actually think you're reporting pretty good performance based on what I have experienced. I know you don't want to hear this, but a 3m boat is far more docile, and a 3.5m boat is heaven. Shorter boats are more difficult to plane, and more difficult to stay on a plane.
3. The tubes, obviously, must be drum hard. If you see any wrinkles or detect that they are moving under your rear, that's a problem. If you have three chambers, pump up #1 and #3 most of the way, then pump up #2 so that everything equalizes when the baffles get pushed aft. Never pump up a single chamber completely or you'll risk blowing the baffle out of the insides.
4. You might want to try to shim the engine up using batten stock or a small piece of lumber. Not all transoms are ideal for a 15" shaft length, and you may be immersed a little more than is ideal.
5. Yes a Doel Fin or similar cavitation/ventilation plate foil will stabilize the pitch angle, but with a pretty big decrease in top speed. If you buy one, I'd get the smallest one that fits.

Chuck

[SteveSadler]

15 kts on a flat bottom 2.6m inflatable is like 30 kts on an inner tube. Far beyond it’s designed speed and ready to throw you over on the next wave. Inflated floors flex and deform the bottoms rocker causing the bow down squirrelly handling

[mikedefieslife]

Originally Posted by SteveSadler

15 kts on a flat bottom 2.6m inflatable is like 30 kts on an inner tube. Far beyond it’s designed speed and ready to throw you over on the next wave. Inflated floors flex and deform the bottoms rocker causing the bow down squirrelly handling

It should be well within its designed speed considering it's rated for an 8hp engine.

My issue is nothing much to do with speed it's to do with not being able to get a bow up attitude, and the outboard itself rising up and ventilating.

Someone had suggested getting some wing type things. I can see these helping the outboard to raise even more quickly further adding to ventilation. It was also suggested suggested that the shaft length might have the prop immersed more than that is ideal. If I raised (lifted up) the outboard, surely that will make things worse as more of it will be out of the water.

I am willing to accept that it's just a **** dinghy. No point buying a new one until after the Caribbean. It seems no-one leaves the Caribbean with the same dinghy they arrived with.

[SteveSadler]

Mike, what I mean by deforming is the fore/aft rocker is no longer flat or convex as designed. Put a straight edge on the bottom fore/aft and you'll see its concave with a load in the dink. This creates a downward angle at the stern which gives too much lift at 15kts.
It is likely why it will plane 2 people as well if it was flat rocker it wouldn't have as much lift or force the bow down. Putting a foil on the motor may help as water passing over the top may have enough down force to counter the excessive lift. You may think about getting a bigger Rib with a 15 Yamaha enduro. They tow better and fit on the bow right side up with engine on it.