Engine height on a Aerotec 420

[C-NUMB]

Hi
In most cases when cavitation-plate is in level with the bottom is a good alround instalation height for the engine. The bottom/transom shape on the Aerotec 420(guess same also on the 380) is not a traditional straight V, so my question is: Has anyone here played with the engine height on this sib type and lifted the engine so that the cavitation plate would be even with the bottom? By default the cavitation plate is lower.

[Locozodiac]

Originally Posted by C-NUMB

Hi
In most cases when cavitation-plate is in level with the bottom is a good alround instalation height for the engine. The bottom/transom shape on the Aerotec 420(guess same also on the 380) is not a traditional straight V, so my question is: Has anyone here played with the engine height on this sib type and lifted the engine so that the cavitation plate would be even with the bottom? By default the cavitation plate is lower.

What engine do you plan to use, a Tohatsu 18 HP, 4 strokes engine ?

[C-NUMB]

Yes, that was the original plan. However it's still in the box, and having second taughts, would get a old 2 stroke 25/30 hatsu with the same weight and guess much better speed/handling. In fact alway's enyoyed the simplicity of two stroke engines.

I know tohatsu specks the cav plate to be 30-50 mm below the bottom, but in tha past used to have a couple of boats always installing even with the bottom without any problems. The transom is 500 mm. A long 25/30 cav plate to transom is 560 mm and on the short one its 435 mm, so some work ned to be done anyway.

Will post a picture of th transom/bottom shape litle later.

[C-NUMB]

Here a few pics, found on the net. The V shape of the transom is kind of rounded. Have no idea how this slighly odd shape impact on the engine instalation/height. Reading this forum seams like at least the 380 performes rather well. It looks like the bottom before the transom is kind of V but then gets rounded closer to the transom.

[Locozodiac]

Originally Posted by C-NUMB

Yes, that was the original plan. However it's still in the box, and having second taughts, would get a old 2 stroke 25/30 hatsu with the same weight and guess much better speed/handling. In fact alway's enyoyed the simplicity of two stroke engines.

I know tohatsu specks the cav plate to be 30-50 mm below the bottom, but in tha past used to have a couple of boats always installing even with the bottom without any problems. The transom is 500 mm. A long 25/30 cav plate to transom is 560 mm and on the short one its 435 mm, so some work ned to be done anyway.

Will post a picture of th transom/bottom shape litle later.

Tohatsus are water splashers if incorrectly installed, put engine trim angle in third hole from transom as to have the engine perpendicular to water, balance mates properly towards bow, full wot on calm waters and see if the water level is passing 1-2 mm under upper small cav plate, that would be ideal position for short and long tails engines.

Happy Sibbing

[C-NUMB]

Thank's locozodiac. Sounds like a good advice to me. With this adjustable set-back plate :

http://www.allinflatables.com/shopping/cook/sb.html" adjusting would be a piece of cake. But guess its a bit overkill on a sib....

Simple version if needing only to lift the engine : http://www.allinflatables.com/shopping/cook/ve.html

[uny1965]

hi all, i recently bought the 380 aerotec with 18hp tohatsu 2 stroke (short shaft). i to had problems with the position of the motor on the transom and agree with locozodiac (put engine trim angle in third hole from transom). i found this to be best, even though i still seem to be getting a lot of water splashing over the transom ,even when the weight in the boat is evenly distributed. (we have to go ashore every 30 mins to bail her out) any body got any ideas ? i have seen a few ideas on the web, this pic being 1 of them, if you look at the transom the owner has added some sort of spray deflector (anyone know where you can buy them or is it a home made job). also one other question (sorry) how has the owner of the boat in the pic been able to add a jockey console to an inflatable high pressure floor ??????

[Locozodiac]

Well, glad to read that third hole from transom performed much better, now you have to correct the splashing issue that is caused by water flow smashing against the exhaust section somewhere inside the splashing parameters. See pic to understand.

This issue is worsened if you have a cut in middle transom, if yes, probably you have a 38 cm height transom, two options, raise transom center height to 40 cm or add a height plate of 2 mm to raise transom to 40 to perform as expected. That's why all our sibs are being factory delivered with 40 cm transoms to be able to use Tohatsu engines on all of them.

The tech explanation is quite simple, water flow should pass under small cav plate when on plane on calm flat seas, with sib correctly mate balanced & inflated (tubes, air matt V keel) use a gauge, water flow must not be passing inside splashing parameters whatsoever, which seems is your case. Fixing height will reduce water drag against tail, unnecessary drying and optimize engine top end speed.

1-2 mm under small plate is ideal, rising engine so that passing water flow is under more than 2 mm will cause prop to cavitate in chopy seas. Adding a spray side deflector won't solve your problem, as these deflectors are usefull when you have water swirling and entering sib at side tubes specially on plane on chopy seas but not water jumping over transom/engine area.

Happy Sibbing

[Easyrider]

When i had my 380 with a Tohatsu 30 2 stroke, i raised it by 25mm(cav plate was level with the keel) by adding a 25mm piece of wood on top of the transom and raised the metal transom plate too.
It totally solved the splashing problem and performed better too.

[C-NUMB]

Thanks Easyrider for information. Think will cut the transom first for getting even with cav plate, can always cut more if need(due to prop cavitate or other reason). How did the 380 handled with 30 hp, plenty of flex on the air floor?