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Old 18-01-2018, 04:12   #16
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Re: Trouble with my outboard height?

Wow! So many replies and ideas!! Thanks!

The engine is a 25, upgraded to a 30, so not a heavier 30 block, if that makes sense. The manufacturer rates it at 30 but doesn't specify an engine weight.

My apologies, I'll take a further photo in the morning so you can see the cavitation plate.

When the engine was lower it ploughed up so much more water... perhaps it's not high enough, but I'm pushing the friendship with physics if I go higher as it feels like the engine will have too much leverage on the transom if I go any higher... if that makes sense. It's the shortest shaft model they had.

It does have a habit of venting/cavitating when I turn. I have to straighten up, back off then fire it up again.

Perhaps it simply is too much horsepower and perhaps a 15hp may have done the job. This dinghy doesn't really "plane" as one normally would, it just zooms off flat and instantly planes. It would need a lot of weight in it to cause it to have to plough onto the plane.

Good point re things hanging off, however the wheels are located well above the water line.

It certainly could be the trim angle, but I'd be chasing it everytime the load in the dinghy changed. Perhaps this sized engine really should be fitted with electric trim and steering etc for best results.

It's gotta be height as the issue, but I think an issue if I go higher. I could perhaps trial one of those stingray plate things.

I'll attach a further pic in the after here in AUS tomorrow.
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Old 18-01-2018, 10:18   #17
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Re: Trouble with my outboard height?

I dont think a 15hp will be good. I had 15 on my 10ft heavy RIB and it wasnt over powered. Later I had the same engine on a new lightweight 9 ft RIB and it was scary wild. But a 15 HP on your large RIB may not be adequate. Maybe 20 HP or so, not sure. It was a real dilemma on the large Honda engine I mentioned earlier. But I suppose it has to be a height, weight, or tilt issue one way or another. Stock prop?
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Old 18-01-2018, 10:35   #18
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Re: Trouble with my outboard height?

Raise engine by 10mm increments with shims. Usually 20mm to 30mm is the sweet spot.

Check keel inflation to verify it is at operational requirements (usually 240 mbar).
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Old 18-01-2018, 11:04   #19
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Trouble with my outboard height?

If it zooms off flat and instantly planes, then the motor need to be tilted.
Even with huge power, a boats bow should rise, then lower as it gets on plane, with a motor tilted too far down it can make the bow dig in too much and can make handling dangerous.
Ideally on plane only the aft part of the hull should be in the water.
Look at that video where the powerboat runs over the fishermen for an example.
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Old 18-01-2018, 13:03   #20
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Re: Trouble with my outboard height?

Most dinghies with the wheels are less than 10ft long. If that is the case here, 30hp puts it a power - to - weight ratio like the space shuuttle. Boatman has the answer. imho
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Old 18-01-2018, 13:46   #21
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Re: Trouble with my outboard height?

And at what speed are you getting the rooster tail and water coming aboard?
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Old 18-01-2018, 17:06   #22
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Re: Trouble with my outboard height?

I haven't measured the speeds sorry so I can't tell you.

I'm beginning to think that the set-up is just a bit less than average. The dinghy goes well, nice and fast, carries a lot and is pretty seaworthy. It's not the end of the world... I'm very happy with it overall, it just would have been nice if it was *perfect*.

Based on my photos, it's looking like the leg is too long for the transom... but based on my old Zodiac, it looks somewhat similar and that was a great boat. It didn't need the plate added to it, but someone gave it to me so I fitted it.

You may note on the black engine that the trim is set right to the lowest setting, however lifting it up more than 1 notch makes the prop vent regularly when heading into a chop.

As someone pointed out above, if the trim is too low, then handling will be affected and yes, it does behave like it's bow is too low and it doesn't sit super stable on bumpy or even flat water at over medium speed.

The red one is 3.5 metres and that needed every bit of 25hp. The black one is 4.3 metres and it feels like it would be fine with a 15-20. Weights are similar for size difference.

Re the comment regarding the wheels - they were fitted to assist at the boat ramp if it's really busy. Down here on a nice day you could be waiting an hour to get your boat out of the water at the ramp! With the dinghy wheels I get priority and I can then put it onto the trailer in the car park easily enough. They work well although it's a little heavy in the front, I can do it alone without too much trouble









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Old 18-01-2018, 20:43   #23
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Re: Trouble with my outboard height?

Hard to say because of perspective issues, you might go up another inch or so, but you're way over propped...trimming out might help a little too...
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Old 18-01-2018, 21:11   #24
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Re: Trouble with my outboard height?

What makes you say I'm "way over propped?" It's super responsive and when I say that, I really mean it, like rocket ship acceleration, the boat is light and it revs out easily to max* revs with seemingly no load. It's just got a roostertail which is possibly holding it back a little for speed and stability. I know a lot about props. I've got a variable pitch prop on my 4 seat aircraft that I built.

* This engine doesn't have electronic monitoring like our e-tec 120 so I can't confirm max rpm are being reached, I'll have to fit a tiny-tac, however I'm confident it's close to max based on experience.

Photo taken from level with transom bottom.

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Old 19-01-2018, 04:41   #25
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Re: Trouble with my outboard height?

Pardon me, I should have said 'appears way over propped'.

After looking at it again, I should have said it 'appears mis-propped'.

How fast do you want to go? Hard to know if it reaches full rpm without a tach, so it's hard to know if you're what is generally considered overpropped.

Looks to me like you could raise the outboard a little more, increase the diameter and decrease the pitch of the prop, and trim it out a bit, but I'm not there...
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Old 19-01-2018, 06:47   #26
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Re: Trouble with my outboard height?

Have you inquired at any marine dealerships regarding this issue? It does appear that the motor shaft is too long but that could be due to the angle of the photos.
Try an online search on how to set up an outboard motor for proper performance. As far as "over-propped" goes, I doubt if that's your problem, my experience with props is pitch either affects your top-end performance, low-end (hole shot) or somewhere between the two. Kind of like your sport plane, if I remember correctly, dial it to the firewall at take-off and back it out for cruise. I'd be surprised if your prop is the problem.
I sailed with a person whose dinghy was set up for a long shaft motor, he had a short shaft. He could never go much more than 1/4 to 1/2 throttle without severe cavitation issues. I suggested either switching to a long shaft motor, having his changed to a long shaft, or cut down the transom to accommodate the short shaft engine he's got. He opted for the cheaper route, shaving a bit off the transom. The results, he's more than happy he did it, the motor performs as it's supposed to!
I wouldn't mess with the trim angle too much, the transom angle is set by the manufacturer and I would think falls in line with the engine manufacturer guidelines. Your owners manual (for the motor) should have a chapter on how to properly mount and set up your motor. If not, I'm sure you can find that information online.
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Old 19-01-2018, 07:49   #27
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Re: Trouble with my outboard height?

If you don't mind, pm me with your motor and dinghy data. Don't know if the motor being a two or four-stroke makes a difference but it may because of motor weight and rpm's produced.
I live near the manufacturer that produces your motor and can contact the on-site techs who can hopefully help with your issue and eliminate much guess work.
Cheers, Steve
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Old 19-01-2018, 13:34   #28
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Re: Trouble with my outboard height?

Thanks for the offer however there's no data on the dinghy. It's a Chinese copy of a Zodiac, so that's probably half the problem? All it says is 1087kg max 30hp/22kw. Transom is 15".

Engine specs are 51kg, max rpm 5850, fuel 13lph, 430cc, TRANSOM HEIGHT 17.1 - 22.1.

Aaaaahhhaaa... amazing what you find when you read the manual! It's definitely the shortest shaft available for that model, just a tad long. Just looking at my old Yamaha, I can't help but feel it was a bit shorter.

I'll go a bit higher and report back as soon as I'm done.
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Old 19-01-2018, 14:09   #29
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Re: Trouble with my outboard height?

Just spoke with a technician regarding your issue. He couldn't give me much help without having the serial number of the motor but, did indicate that it could be a combination of several things: too long a shaft, prop pitch (which I didn't feel would be an issue but definitely could be) and your set up or trim angle were three items you may want to look at, for starters.
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Old 19-01-2018, 14:10   #30
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Re: Trouble with my outboard height?

Chinese copy of a Zodiac?
Not that China is unique, but their "copies" and "ratings" often are fanciful about details. It is possible that the copy is not faithful, and that 30hp is indeed simply too much power for the boat. Not saying you should give up--just bear that in mind as possible.
Chinese LED lights are often "rated" at 2x-3x the wattage they actually consume. Who's to say Chinese horses aren't, well, similarly mismatched?
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