Excess spray from outboard on SIB

[chriswood]

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

[jyasaki]

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

[chriswood]

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

[jyasaki]

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.

jky

[chriswood]

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

[HDAV]

40mm seems quite a lot

[chriswood]

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?

[HDAV]

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)

[martin_smith]

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?

[bartiny]

Originally Posted by chriswood

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

[chriswood]

Once again thanks all. Yep Martin it was your thread that got me thinking and like you say never thought it was a problem until it was mentioned. (Cheers mate :thumbs) You like me, you have prob got a short shaft - but like mine its 17 inch which is just that bit too long for most Sib transoms. I think like you I will raise a few cm's maybe more and see how it rides (don't really want to drill more holes if need to but if it makes a big difference will do).

Re keel inflation I don't think it's that as I have been under inflating keel and still had this issue (had other issues too!!). Reading all different internet articles it seems that the best height for the anti vent plate is around level with bottom of transom or even higher so you like me are well below that by 5cm. Interestingly some articles were saying that although in a similar situation to ours they raised the engine and then had more problems so it may not solve all the issues. I think it just giving it a try and see what happens.

I'm learning that boating is just one big learning curve that prob will never end!!!!

[paddlers]

Couple of photos admittedly not very good showing my out of the box 3.5 AE Honwave & 20 hp Tohatsu shortshaft. Absolutely no issues with splashing with this setup either solo or with 5 people aboard.

I hope this helps!

[martin_smith]

Chris...ha ha I am also discovering its a very steep learning curve!!!

Lookin at the pictures of Paddlers boat, that would appear to be the way forward. I mentioned the keel pressure (over inflation) cause I had mine pumped up very hard whilst I was away a week ago and was just wondering if over inflation could be a contributing factor.

[paddlers]

With regard to pressures they were spot on prior to putting it in the sea & I didn't top them up as I'm more concerned about the thing blowing a seam when it's sat on the beach in the sun which it did for two weeks when not used !!!!

[chriswood]

Helpful pics Paddlers - reinforces the thinking that level with bottom of transom is where it needs to be. I also worry about too much pressure on boat so end up inflating then letting air out and then inflating again etc... Pain in the butt. All Ribs should have pressure release valves as standard - save the worry!!

[paddlers]

I agree about the pressure release valves. If I lived near someone who could fit them I'd have it done, the tubes feel rock hard when they get the sun on them and it's a pita taking the pressure gauge/pump every time you want to use the boat.
For the relatively small cost to the manufacturers and the catastrophic cost of a blown seam when on holiday you'd think they'd be standard !

[jyasaki]

Originally Posted by paddlers

With regard to pressures they were spot on prior to putting it in the sea & I didn't top them up as I'm more concerned about the thing blowing a seam when it's sat on the beach in the sun which it did for two weeks when not used !!!!

It can be an issue. I partially deflate my RIB when it comes out of the water on a sunny day.

I have an on-board electric pump, and top the tubes off to working pressure after splashing the boat.

FWIW, I bought some OP valves a while back, but haven't gotten around to having them fitted.

jky

[paddlers]

JKY I've thought about these quite a bit but I'm a wimp when it comes to hacking into a perfectly good tube to fit them!

They would solve the main issue which to me is pumping the boat up to pressure on cold water then bringing it ashore which we do a lot to let the kids snorkel/fish from the shore. To that end I content myself with hitting pressure ashore before we set off and tolerate the cold water drop in the knowledge I can to some degree relax about the thing going pop!

Not that it gets that sunny but the tubes do seem rock hard in no time!

[bartiny]

Just take a deep breath, Stanley knife, small pair of scissors, and fit these;

NEW A 6 Leafield Air Pressure Relief Valve Boat Rib Inflatable Marine Sib PRV C7 | eBay

Took me half an hour to fit 3, once I had plucked up the courage to start

[Marcusredford]

I am having the same issue with mine, a 40 Marina where at high speed getting a lot of spray over the transom. I'm a complete newbie, can you explain how I alter the height and trim?

thanks