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Old 01 June 2015, 00:50   #1
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Move up from 20hp to 30hp?

We had our second fun excursion on our 15' SIB but still having issues with figuring out the trim for the 20hp 4-stroke.

First time out ventilation/cavitation was horrible. As soon as it would start to plane it would happen excessively. After what was still a fun day, I looked into solutions. Measured the cavitation plate and saw that it was 1 to 1.5 inches below the transom and keel like it should be. Went out for a second time but pumped up the keel and tubes to their max after getting in the water. I also added a hydrofoil to the engine. Much better. Still had issues with it porpoising even with the tilt pulled all the way in. Porpoising was minor with it oscillating about every 2 seconds regardless if it was calm or rough. We moved weight around and no matter how we distributed, it still did it consistently. Family was doing fine but it annoyed the hell out of me, especially since I wasn't finding a cause or solution.

The 20hp got all 5 of us up to 16mph max which was expected and the family enjoyed it like we thought since all we did was sight see and tour around the lake. When I head out with a friend though, I'd like to go faster. Found a deal on a 30hp 4-stroke. My worry is the performance. Would any of the issues I currently have be amplified with more power?
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Old 01 June 2015, 02:34   #2
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That performance sounds very good for five people in a 4.5m SIB with a 20 - how heavy are you and your family and what other load do you carry? Might be worth trying with just two of you and the 20 first.
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Old 01 June 2015, 06:59   #3
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I find that on certain SIBs they porpoise at certain speeds, but once above those speeds they're fine. There's lots that can affect it, the angle of rake on the prop is often suspect, on thundercats at least, probably not the case if you have a standard prop.

Have you tried the boat pumped up to the max but without the hydrofoils?
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Old 01 June 2015, 08:21   #4
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Thanks for the responses. My wife wouldn't want me to reveal her weight but....with us two and three small children, we're at about 560lbs all together plus minor gear including 6ga tank, small cooler and small cooler.

I've been wondering about testing it like you both have said and it's a good thought. We couldn't take it with just two of us, the kids would be disappointed. I may be able to take it out myself while my wife watches them on the shore just to try it out.

I was thinking the same thing about testing without the hydrofoil attached. I do think it has made the most improvement though. Even with the tubes pumped up, any other trim setting other than all the way to the transom, would cause ventilation every now and then. It got worse the more it was moved out.

I was kindof wondering if the porpoising was due to the boat being just on the edge of settling into a plane at the top speed we were able to reach. Just a little more and it would probably level out.

It was on the rare side for us to have it open throttle. We were usually cruising slower touring around. Reason we got the 20 was more due to the handling of it since we fold our boat up after each use but we knew it would be underpowered. Now that we have our act down and figured out some lifting straps which make it easy, she's ok with stepping up the size. Will 30 make that much different with our size and all of the weight? If we could hit 20mph, I think the boat would settle in on it's ride better.
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Old 01 June 2015, 08:48   #5
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As you'll know it's all about compromise. 560 lbs is three 12 stone adults + a small child (easier to visualise this way) so quite a load on a boat that size. Yes, the 30 will be (obviously) more powerful but will weigh a lot more and be a PITA to carry and handle + it will drink a lot more per hour at WOT although you might not need to thrash it to perform as you need compared to the 20 so difference may be much smaller.

However, it will help a lot with the performance being 50% more powerful. If in any way possible the best solution would be to trial it on the boat.
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Old 01 June 2015, 14:14   #6
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Originally Posted by Max... View Post
Yes, the 30 will be (obviously) more powerful but will weigh a lot more and be a PITA to carry and handle + it will drink a lot more per hour at WOT although you might not need to thrash it to perform as you need compared to the 20 so difference may be much smaller.

However, it will help a lot with the performance being 50% more powerful. If in any way possible the best solution would be to trial it on the boat.
Not really big negatives for us. The lifting and moving around of the 20 is simple when using the shoulder straps between two people. Puts all the weight on the shoulders and down through the legs instead of arms and back. The extra 30lbs for each person a 30 would have wouldn't add much. I can't see fuel being an issue. Our 2nd trip out was for 6 hours continuous and we hardly used any. We were actually expecting the 20 to use a lot more than we did.

I am more worried about the additional weight at the transom and how it might affect the ride, planing abilities, etc. Would love to try one out but I don't think it's possible. Next step will definitely be to try it out myself and possibly without the hydrofoil to see what it does.

Thanks for the input, we appreciate it.
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Old 27 June 2015, 19:15   #7
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Got a 9 pitch prop and took it out again. Performance improved some. In calm water we held 18 mph without porpoising and later in the day, in a heavy populated lake, we swung between 15 and 17 mph with chop. Engine sounded less stressed.

I purchased a tach but couldn't get it set right for a 4-stroke. It quit after about 3 hours into our trip. Can't wait to return it. Was looking forward to calculating the slip on the new prop.

Wife was fine with our peak performance and really not wanting to go faster so I'm good but I'm thinking about a 8 or 8.5 pitch prop because I don't think the engine's RPMs are up enough into the recommended range. I'm wanting to minimize the wear and tear on the engine for longevity.

Looks like the 20 will do us fine so just a little more fine tuning and simply enjoy it. This trip was exceptionally fun for everyone.
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