Engine Height question

[dreemon]

Hi to all, been doing some reading in old posts about engine heights, I'm trying to get the cav plate to be just on top of the water on the plane, right now when i look over the transom i can see the backbone above the plate, and cav plate just under water about 15mm?

to raise the engine anouther 15mm, "a 40 hp tiller all manual and on a 410 Brooker tinny," would mean a total of 65mm above the top of the transom !!, does this mean I need to have 50 x 50mm alloy welded on top of the transom just to get the screw in clamps and bolts to fasten the engine securely?

anyone else had to raise there engine up this much? at the moment the cav plate is right in line with the bottom of the hull (not the extruded keel) thanks for advice

[dreemon]

supose no one worries about drag on the leg.......

[ashh]

hi, this looks like it was just listed and may be exactly what you are after.
http://www.ausfish.com.au/vforum/sho...d.php?t=134198

[FNQCairns]

Yeah leg drag is a big deal IMO although you are restricted by just how much (alot) a aluminium pressed hull lifts as it travels faster. At what speed are you viewing the vent plate?

The rule of thumb is to keep going until it does something you do not like - too much ventilation on a turn (of coarse no use doing a ski type turn to test if you never do that normally, the test should suit your normal style) or if it ventilates when travelling in the lumpy stuff again as per your normal driver behaviour.

Your comments above is the exact reason a foiled pressed hull can never work well, it will more than most probably never clear the water unless flat strap, everywhere else it's simply making the setup a dog due to the extra drag induced.

cheers fnq

[dreemon]

Hi FNQ, when trying to look at plate I'll be going close to top speed, and because i get cavatation even when not making a hard turn is why I'm thinking the water is getting airated, I'll just try the engine up a bit more, if no good i can bring it back down, thanks

[FNQCairns]

But is the ventilation a problem to you in your normal use of the boat? hulls with extruded keels can force turbulence/air at the prop on turns, give and take is where it's at while keeping an eye on what YOU want to achieve.

If you came for a ride in my pressed tinny, it ventilates on turns at speed and not hard turns either just easy sweeping ones.

It will also grab lots of air travelling straight at cruising speed over easy half meter waves causing a rev up and a loss of a km/h or 2 until it bites fully again or on occasion I need to back it off a bit as it is telling me to slow down a little, it's all no big deal.

Mine engine is high enough to loose a little water pressure at near wot.

Horses for courses but tuned to be the most efficient it can possibly be for ME and my use.

Good luck in the raise.

cheers fnq

[pedrodepacus]

hey mate does the boat have an external false keel ive seen some occasions where they can cause the boat to cavitate

[dreemon]

Yes pedro it does have an extruded keel that is tappered at the last 500mm at the rear, and FNQ by the way you discribed your tinny mine is very close to the same, can you see the top of your plate or is that not a concern? i'll just see what it does up a lilll more, and thats it... I only want it to be set to best all round. thanks again

[FNQCairns]

Seeing the top of the plate is not a concern, just aim for the highest you can get it on the water following what suit YOU and no-one else the best.

In theory if you knew the prop suited the boat judged by your WOT rpm and speed then you may be able to see visually the vent plate at this engine height and have less cavitation at the same time or it could be the converse.

Keep going up until the ventilation doesn't suit your style of driving and you have done as good as you can do under the circumstance.

Mine I can see, it sits way up above the water and exposes water inlets to some degree when near wot but every boat is an individual and cannot be a copy of another when setting it up.

For instance the next step up on mine would be toward a race trim surface piercing set-up where height of engine is measured from the centre of the prop shaft in fractions of a mm when measured even with the bottom of the hull, that would never work unless I stripped it of all weight, you could do that also but whatever you do has to suit you and your individual boat as is when tested.

cheers fnq

[pedrodepacus]

hey dreemon if you do decide to have the engine at a height where its not very well supported i think you were saying it was 65mm above the transom now i would be running it to make sure your happy with it then getting something welded in to give it the support as it seems along way up to be unsupported

[bigtez]

I had a similar issue in regards to not having enough transom height to get the motor where I needed it. I have solved that issue with a cast alloy jacking plate from basspro.com.

[dreemon]

Pedro welding is what I was thinking 50 x 50mm but when it comes time to buy anouther engine (if mine blows) i'd probly be better off with the jackplate bigtez posted, I did some reading and found out that jap outboards have legs that may be one or two inches (25 - 50mm) longer than the US-built competion and mine is made in japland, I have raised it to try it at 63mm abouve the transom and see how it goes, and now the screw in clamps are sitting halfway on the top lip, ( I have an alloy block supporting it temporarly) strong for a test height, and a chain..... we'll see how it goes when the weather and work let me, thanks heaps

[pedrodepacus]

hey mate i understand the different leg lengths we see it all the time with the different engines if you do need a piece welded in just send me a pm i can do that for you no drama .did you get the rig from marine tune

[Lancair]

Dreemon, What is exactly the problem? Is it the ventilation during turns, but going straight its fine ? Is it that you cant see the cav plate above the water whilst on the plane ?
Do you have tilt n trim ?

I have an Etec40 w/t,n,t on stessco catcher. If its trimmed for going flat out, ie raising the motor until it just starts to cavitate or ventilate what ever you call it, then trim back down just a tad, I will get the highest speed I can out of the boat, but try to turn more than a slow drifting heading change and it'll cavitate, if I trim it down a bit more I can turn harder without cavitation. Trim in more and I can turn so hard people not hanging on or any gear not bolted down will go flying. Top speed suffers as you trim down but turning ability is greatly enhanced. My cav plate has a permatrim fitted, but that was to help early planing when solo. It sits dead level with the bottom of the boat, not the small (3/4 inch) keel, when the motor is trimmed so a straight edge sits parrallel to the cav plate and boat bottom. When trimmed for flat out on the water I can see water going over the top of the cav plate by about 1/2 3/4 inch.

If you dont have tilt n trim youll have to find a trim spot that gives the best of both worlds.

Andrew

[dreemon]

OK,.... Lancair, would love tnt but all manual for now to help keep fit, I will take the boat out for a burn aft work tues, and see what the new height does, it goes great in a strait line like yours and FNQ, and slow turns cavatates with trim out, but like yours,, trim down lose speed gain steering response, i can deal with all that but like everyone , looking for the max performance the engine/boat will put out, , if it goes a lill better on this test , I'll leave it there ,,, hull fr marine tune


and Pedro.. thanks heaps for the offer,if it works good at this OR the other height I will ask for your help getting the welding done, are you on the goldie? If i took a pic now of the set up it would raise some eyebrows Cheers for beers