Engine height on a Aerotec 420

C-NUMB · 06 January 2010, 20:45

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 · 06 January 2010, 22:27

Quote:

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 · 07 January 2010, 19:57

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 · 07 January 2010, 21:11

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 · 07 January 2010, 21:30

Quote:

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 · 09 January 2010, 12:21

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 · 13 January 2010, 23:08

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 · 14 January 2010, 18:15

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 · 14 January 2010, 19:14

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 · 14 January 2010, 20:07

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?

uny1965 · 14 January 2010, 21:59

thanks easyrider and locozodiac for all your help. will definitely be raising the transom height when the weather over here gets warmer. just googled bombard aerotec and found these 2 images. (original bombard transom and modified version) hope this helps other bombard owners. 1 more thing :- does it matter if the wooden block is 20, 25, 30mm, is it an exact science ?

Locozodiac · 15 January 2010, 00:29

Quote:

Originally Posted by uny1965

thanks easyrider and locozodiac for all your help. will definitely be raising the transom height when the weather over here gets warmer. just googled bombard aerotec and found these 2 images. (original bombard transom and modified version) hope this helps other bombard owners. 1 more thing :- does it matter if the wooden block is 20, 25, 30mm, is it an exact science ?

20 mm is a good starting point to solve splashing issues forever, if still splashing a bit try 25 mm, must screw transom front and rear plates to match this new position. To understand what you must do, see pics. If the tech procedure is hard work, try a 20/25 mm metal plate and secure to middle transom to compensate.

Happy Sibbing

Easyrider · 15 January 2010, 07:14

I dont think you will go wrong with 20 or 30 mm height- i forgot to add, you also need to raise the outer transom board up too(its only screwed on)-the extra piece of wood goes between the forward metal plate and the outer ply board.
It handled extremely well with the 30(much much better than the 15 it came with), the only time i ever noticed the floor really flex was if i was on flat calm water as it built up some form of pressure wave beneath it, never did it with if there was any chop. It used to do about 30knots

John Kennett · 15 January 2010, 11:16

On my 380 Aerotec the previous owner had fitted a piece of wood along the top of the transom to raise the engine height by about 25mm. It works fine with the motor at that height.

I took it off to try it as standard and the boat suffers badly from spray at speed - I'll be putting it back next time out.

Locozodiac · 15 January 2010, 12:17

Just to take the nail out and for tech data, what is the total height of your wooden rear engine support block if happens to be a square one, see pic. Forgot to mention that the height you must add to transom (20-30mm) to raise engine to avoid water splashes specially if using Tohatsu engines, will depend on tube diameter, as you have sibs with 40/42/45/50 mt. diameter tubes, although not rocket science, has some precise science to it. In my case, needed 25 mm transom raise because my sib has 45 mt tubes.

Happy Sibbing

Nick Hearne · 05 February 2010, 08:56

I had to do the same to my 380, it worked a treat no more water coming over the transom!

uny1965 · 05 February 2010, 17:53

hi nick, just looking at the pics of your boat, seems like a lot of people have been having trouble with the aerotec transom/engine height. am interested to know what you used to raise the engine height, is it just a piece of scrap wood you had lying about or is it something you bought specially and also what size is the block. most people seem to agree 25/30mm block is best.

Nick Hearne · 06 February 2010, 16:12

Hi it was just a bit of wood I had lying around it would be between 25 to 30mm & I took off the ply plate off the transom & moved it up to support the new block & painted it black! Works a treat.
Easy mod & makes a world of difference.