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Old 07 October 2013, 20:19   #1
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Hydrofins on a 4hp?

With the restriction in Hong Kong of horsepower to 4 for anyone without a boating license (and the fact it's the biggest size I want to lug from my village house down to the water everytime I want to go out on the water), I was doing some research on how to make outboards "go faster".

One of the things I saw was "hydrofins"; did a search on this forum but didn't turn up anything. Basically plastic fins that are attached to the outboard near the cavitation plate to give some lift. All the ones I found online were for 25hp and above engines. Is there any companies making them for small 4hp outboards? Would it make any difference at all with such a small hp motor?

I'm using it to push an inflatable cat style boat; waiting for a new one to replace my storm damaged one. The new one will be a 3.3m aluminum floored zapcat style boat. 4hp is probably just going to make it putt along, not even sure if it would plane? So I thought if hydrofins would help I'd definitely go for it.

Any thoughts on other ways to make a tiny 4hp go faster is appreciated
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Old 08 October 2013, 02:22   #2
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Hydrofins won't make the boat go any faster. What they do is add ''grip'' to the back of the boat to make it plane at lower speeds.
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Old 08 October 2013, 03:01   #3
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Also unsuitable as you are lugging the engine around. They are fixed in place and not easily removed without disassembling one/both. I bet within 5 paces you will wish that you had never fitted them as they will constantly get in your way.
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Old 09 October 2013, 11:56   #4
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As henry said, the fins will reduce your top speed, not increase it (more drag = less speed.) They're supposed to increase "lift" to get the boat up out of the hole a little quicker.

With a 4 hp motor, I don't think you're going to have enough power for that to make a difference; and if, in the off chance that it does, it will be offset by the increase in drag.


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Old 09 October 2013, 12:43   #5
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Quote:
Originally Posted by jyasaki View Post
With a 4 hp motor, I don't think you're going to have enough power for that to make a difference; and if, in the off chance that it does, it will be offset by the increase in drag.

jky
Precisely. A 4hp motor would not benefit from fins. As many models of 4-6hp share the same block and are the same weight you could always invest in a sticker and rebadge a 6hp as a 4hp.
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Old 09 October 2013, 13:15   #6
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Need someone to explain that to me. If you do 12 knots one up with a 4 hp and 12 knots with a 10 hp two or thee up, or 12 knots with a 15hp five up, why would you fit fins to the bigger engine only?
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Old 09 October 2013, 13:54   #7
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A boat fitted with a 4hp is unlikely to plane at all so hydro fins, designed to aid planing, wouldn't make any difference. In fact, the drag effect of having more gear under the water line will make it slower.
A larger engine is at least capable of getting a boat on the plane, but it might not with extra load if only capable of displacement speed, in which case hydro fins would again not help.
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Old 09 October 2013, 14:43   #8
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Cat type inflatables, dont like small engines.
My opinion is that if you have to fit a 4hp , then with the fins you will be better so you will go a "bit" faster !!
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Old 09 October 2013, 17:34   #9
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Some reports of 2.4m v hull which will plane with 1 sitting forward with a 4 hp. My 4hp was on a slatted floor so didn't plane anyway but with the right hull, low weight and right trim should get up on the plane.
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Old 10 October 2013, 04:37   #10
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For info, the OP's boat is 3.3m. A 2.4m tender could quite easily plane, as would a surfboard with a 4 hp motor ;-)
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