Much spray - is this normal ? Honda Air V 3.2

[ruddles]

Hello all
I have a new honmwave (Honda) 3.2 Air V floor with 18hp ss tohatsu. We just replaced our 3.1 m ali floored rib as we could not easilly fit it to the foordeck of our boat.
First I had major problems getting the recommended pressures - .35 bar tubes, .8bar floo sections. Cured this with an electric tyre pump and home brew adapter with accurate pressure gauge in line. now all pressures ok.
Under power there is excessive spray from around the ouboard leg, this never occured on the ali rib, also, the floor flexes as power is increased. On the rib, the anti cav plate was apx level with the bottom of the hull, on the sib, it is around 2 inches lower.
The spray really is bad !!.
I realise the sib is rated for 15 max, but the 3 ponies more shouldnt make any odds, the problem occurs as oon as it comes on the plane and gets worse as revs and speed increase. The floor flexing around 18" forward of the transom also seems to be effecting performance at the top end.
Is the engine too low ?.. I have read of spray problems with the tohatsu due to squarish exhaust section ?.. but as said, it never occured on the ali rib..
Any help or advice you can offer greatly appreciated.
Joe.

[jyasaki]

Sounds like the motor is too deep. That causes the water trying to go around the leg to spray up to either side, and often, back into the boat.

Although a lot of manufacturers suggest the anti-ventilation plate even with the keel, that's usually considered a starting point. Often it can be moved higher.

One way of checking is to take a glance back and down while running reasonably fast; the plate should be at or near the surface. Some people say you can run with it out of the water with no problems. Your mileage may vary.

Luck;

jky

[Locozodiac]

This Tohatsu 18 HP excesive splash issue has been treated full on previous thread/posts, it's not a nomal issue, have passed through this myself, solved for ever. If your transom is under 40 cm height, better modify by raising it. Tohatsu recommends a height between 3 to 5 cm from boat's bottom to anticav plate, as you are using it must have at least 7/7.5 cm, a 2, 2 1/2 difference that will give splash problems. Somebody out there has added a traingular wooden shape splash guard secured with tie wraps, and working ok. Would recommend to use your engine trimed on second hole from transom out and place all weight towards bow, will help slight better, but not solve the general problem.

http://rib.net/forum/showthread.php?t=23192&page=2

Don't lose time & effort trying to modify a cheapo inflatable floor, that is the standard ones compared to more pro ones, like in more costly larger Zodiac for example. Even inflating floor to it's max pressure 10/12 PSI still will flex/rock a little. Is the issue of having portability loosing rigidity. Just deal with it or pass to a hard floor.

Happy Sibbing

[Locozodiac]

Originally Posted by jyasaki

Sounds like the motor is too deep. That causes the water trying to go around the leg to spray up to either side, and often, back into the boat.

Although a lot of manufacturers suggest the anti-ventilation plate even with the keel, that's usually considered a starting point. Often it can be moved higher.

One way of checking is to take a glance back and down while running reasonably fast; the plate should be at or near the surface. Some people say you can run with it out of the water with no problems. Your mileage may vary.

Luck;

jky

JKY, if you have a exposed anticav plate out of the water, will give you excesive engine cavitation specially on choppy waters, engine won't refrigerate well, specially if you have big water intakes, don't confuse small plate out of water with big anticav plate. My boat once on plane has the small plate 2 mm out of water with a raised transom of 40.5 cm compared to previous factory 38.0 cm delivered.

Happy Sibbing

[jyasaki]

Originally Posted by Locozodiac

JKY, if you have a exposed anticav plate out of the water, will give you excesive engine cavitation specially on choppy waters, engine won't refrigerate well,

Not true. At speed (which is when the motor climbs out), there is enough forward speed to drive the suction wave aft of the prop, and enough water pressure from water "movement" (relative to the engine) on top to keep what suction remains, at bay.

As far as cooling goes, well, people run their engines in this configuration all the time on this side of the pond. I assume that you get a wave or somesuch that covers the upper water intake (underside of the cav plate on my engine), but that's just a guess (I'm not sticking my head down there to find out.) I suppose you could ask them.

jky

[Locozodiac]

Originally Posted by jyasaki

Not true. At speed (which is when the motor climbs out), there is enough forward speed to drive the suction wave aft of the prop, and enough water pressure from water "movement" (relative to the engine) on top to keep what suction remains, at bay.

As far as cooling goes, well, people run their engines in this configuration all the time on this side of the pond. I assume that you get a wave or somesuch that covers the upper water intake (underside of the cav plate on my engine), but that's just a guess (I'm not sticking my head down there to find out.) I suppose you could ask them.

jky

The point is that with that (excesive anticav plate out/near water surface) will exceed what the manufacturer recommends 3-5 cm for that 18 HP engine to work well, sorry, was reffering to this particular splash problem.

Dont think "people run their engines in this configuration all the time on this side of the pond". For that to happen will need to raise the transom of any sib about 10 cm more from what's factory standard delivered. That issue must work well in deep hull ribs, racing ribs, hard hull boats, not in sibs.

Happy Sibbing

[m chappelow]

my tohatsu engine is lower in the water and makes bad spray so what i did is to make a shaped wooden fairing with a slot for the gear linkage to move and just strapped it on the leg with cable ties where the flat is on the leg and that has reduced my spray by loads .

[ruddles]

Excellent stuff. In the pic of the towy in the last post, it looks like a long shaft, mine being short shaft has the linkage arrangement just above the top of the higher plate. Not a good design, but I love the engine performance.
Will add the blocks as you suggest, great idea.
Also, re raising the transom height, what is the recommended way to do this please ?
Ruds.

[Locozodiac]

Originally Posted by ruddles

Excellent stuff. In the pic of the towy in the last post, it looks like a long shaft, mine being short shaft has the linkage arrangement just above the top of the higher plate. Not a good design, but I love the engine performance.
Will add the blocks as you suggest, great idea.
Also, re raising the transom height, what is the recommended way to do this please ?
Ruds.

This water splash will vary accordingly to each tramson heights between brands, a 38 transom height will splash more water into the boat compared to a 39 transom height. The lower the 18 engine, the worst. Maybe Chap has his transon higher than I had mine, that's why he only needs the wooden attachment, reading his post he states "that has reduced his spray by loads", so will assume he still has some minor splashes problems.

Raising the transom height will solve the height prob for ever, but it's a more tedious, time demanding job, has to be done by a skilled fiber glass boat tecnician, would't dare to do it myself as you could lost your belowed engine if un correctly done. The procedure is on the thread, entirely up to you if opting for rising transom.

So to understand, see pic transom heights splashing differences. Now with 40.5 cm total height, once on plane, water passes 2 mm down small plate with not single drop splash. Before you add blocks, determine where is the water hitting the engine once on plane. Didn't noticed that M.Chap tail is a long one and that's a big difference between both tails versions. Anyway just curious, what is you transom height from where the engine is placed in direct line to bottom of sib in cm ?


Happy Sibbing

[ruddles]

Hello Locozodiac, thanks for the valued input.

I will measure the transom height and let you know, but certainly the engine IS too low. On the rib, it had the anticav plate level with the very bottom of the vhull rubbing strake. absolutely no splash at all.
Yes, I am sure you are correct that transom height is the cure. On my honwave, the transom is composite, the best way looks to be to make a 1.5mm stainless plate in a u section to fit over the transom extending down to below the outboat fixing plates, the fixing plates then fitted to the stainless, I will get the welder out . That should be a neat looking job. I will add an appropriately sized teak block under the stainless above the transom. Another job for the winter sunshine ho hum .
It was a nice job you did on your transom by the way, looks good.
Ruds

[Locozodiac]

Originally Posted by ruddles

Hello Locozodiac, thanks for the valued input.

I will measure the transom height and let you know, but certainly the engine IS too low. On the rib, it had the anticav plate level with the very bottom of the vhull rubbing strake. absolutely no splash at all.
Yes, I am sure you are correct that transom height is the cure. On my honwave, the transom is composite, the best way looks to be to make a 1.5mm stainless plate in a u section to fit over the transom extending down to below the outboat fixing plates, the fixing plates then fitted to the stainless, I will get the welder out . That should be a neat looking job. I will add an appropriately sized teak block under the stainless above the transom. Another job for the winter sunshine ho hum .
It was a nice job you did on your transom by the way, looks good.
Ruds

Hi Ruds

You are welcome, definitely fixing a stainless steel plate would be much easier job than to raise the transom. Anyway check the toh parameters and begin with the lowest 5 cm from anticav plate to sibs botton (end of transom).

Happy Sibbing

[Locozodiac]

Forgot to mention that Toh's 18 will gime more splash problems on sibs with 40/42 tubes with 38 transoms than 45 tubes with 38 transoms, least of all splashing probs is on larger sibs with 50 tubes with 38 transoms as the engine will be more out of water than on previous 40/42/45 tube sibs. This splash prob is a exhaust design prob, compared to, yamies has more round exhaust tails than rectangular Tohs tails, so keep in mind the manufacturer recommended heights from boat's bottom to anticavitation plate for best sib performance and fuel economy.

Happy Subbing