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Old 27 August 2014, 17:33   #1
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Excess spray from outboard on SIB

I didn't think I had a problem until I read a thread from a few days ago re excess spray from outboard (as a first timer thought it was just normal) - so I went and looked at my set-up. I have a 380hd Ali floor Seapro SIB with 30hp 2 stroke tiller outboard. The boat transom height is 15.5 inches and length of outboard (bracket to anti vent plate) is 17inches so anti vent plate is 1.5 inch lower than bottom of transom.

Having the boat out for the first time last month overall went great but had some issues 1) excess spray, some of which coming over transom 2) on occasion had to fight the tiller on turns (is this normal?) 3) when loaded (2 adults and 3 teenagers) on occasion boat would reduce speed, floor would lift and boat would come off plane. Tried different trim and passenger positions which did seem to help. I did post re 3rd issue and after replies came to the conclusion that it was a keel pressure issue as only had 0.2 bar instead of 0.45 bar which was causing air to get under keel and lift the floor.

My concern now is as follows: Is the outboard too low and do I really need to raise it. Will this resolve the spray problem and poss have caused the other problems too. By raising the outboard will it dramatically improve situation re spray and give better performance or just slightly improve it? Reason I ask, I have drilled holes etc and don't really want to drill more. Boat seems to perform well so wondering if it's worth worrying about the set-up. Would be keen to hear from other Sibbers with your experience or thoughts on this one. Cheers Chris
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Old 27 August 2014, 20:11   #2
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I would suspect your spray problem would be reduced if you raised the motor (has worked for a lot of people in the past.)

Keel pressure (and tube pressure) will have a pretty big effect on performance; you'll get weird problems (like you mentioned) running underinflated - pressure gives the rigidity the boat was designed to run with.

jky
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Old 28 August 2014, 02:40   #3
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Thanks Jky. Do you think it would make a big difference raising the engine i.e reduce all or just some of the spray. Also read that it can improve performance but once again would this be a little or a lot? Only want to move it if it's really worth it. Cheers Chris
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Old 28 August 2014, 11:15   #4
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It's a balancing act. Raising the motor will reduce spray (how much depends on your boat and motor configurations), reduce drag (less frontal area presented to the water stream), but will also increase the chance of ventilation. Your ideal position will end up being a height that makes all 3 of those acceptable in most conditions.

The only way you're going to be able to tell if it helps to a degree that is beneficial is to try it.

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Old 28 August 2014, 17:27   #5
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Thanks once again Jky in taking the time to reply. Think I'm going to raise the outboard 4cm using a shim to bring it level with bottom of transom and go from there. Also ensure all tubes and keel are bang on correct pressure. Hopefully should see improvement. Appreciate your views. Cheers Chris
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Old 28 August 2014, 18:28   #6
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40mm seems quite a lot

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Old 28 August 2014, 18:38   #7
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Transom is 15.5 inch and outboard is 17 inch. If moving up 1.5 inch is this too much - Is outboard too long for boat?
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Old 29 August 2014, 03:18   #8
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Sounds like a 15" shaft would be ideal ( is that 15" to the bottom of the stern) never had spray coming up from our 25hp short shaft (iirc) boat is 150 miles away)
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Old 29 August 2014, 05:40   #9
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It was probably my thread that you read to start with. I own an excel 430 vanguard (Alu floor) with an evinrude 30hp (68 kilos). My boat planes easily and there is no cavitation. I have the engine tucked right in as close to the transom as it will go. I only realised how bad the spray was from my boat after recently being on hols with two others with sibs and watching there boats going at full chat - virtually no spray whatsoever. Mine is like a fountain in Trafalgar Square!

The ventilation plate is just about 5cm below the hull which according to evinrude is the absolute limit. I am going to raise it about 2cm to see if that makes any difference.

I have had one other thought that might seem a bit ridiculous! Is it possible to have the keel OVER inflated. The boat has a very deep V and I was wondering if the extra buoyancy from over inflation could cause the stern to sit too low in the water?
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Old 29 August 2014, 06:09   #10
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Quote:
Originally Posted by chriswood View Post
Thanks once again Jky in taking the time to reply. Think I'm going to raise the outboard 4cm using a shim to bring it level with bottom of transom and go from there. Also ensure all tubes and keel are bang on correct pressure. Hopefully should see improvement. Appreciate your views. Cheers Chris
Just do as you have said here, have a play with it, only you will know if it worth it or not after doing the testing.

Plenty of Sibbers and Ribbers have had to play arround with motor height and trim settings.

Just my opinion
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