Outboard Auxillaries - How Deep ?

[Polaris2]

Hi from Adelaide, South Australia.
I have HitchHiker 37 catamaran. John Our boat has 2 pods under the bridgedeck that take Yamaha 9.9 high thrust motors. Its time to replace these and we are keen to use a model that has electric tilt, as it can be tricky manually raising and lowering motors under the deck. The clearance between the 'transoms' on our motor pods and water level is 350mm (14 inches). We currently have extra lomg shaft motors that put our props well down in the water. However the new electric tilt motors (Yamaha 9.9 GEPL) don't come in the extra long shaft. That means we will have just 187mm (7 and a half inches) from the water surface to the top of the cavitation plate. The pods are sited about one-third of the boat length from the stern, so the clearance is reasonably stable, especially compared with a stern mounted mnotor, but is 187mm enough? The Yamaha salesman doesn't seem to have a clue. Former used car dealer I suspect. But this is a $10K investement here...Anybody have any ideas?
May all your destinations lie downwind under a blue sky[IMG]file:///C:/Users/User/AppData/Local/Temp/msohtml1/01/clip_image001.gif[/IMG],
Polaris

[amytom]

Yamaha does make an extra long shaft with powered tilt and remote kit. I'd look around at other dealers and see what they have to offer.

[fgd3]

Originally Posted by Polaris2

That means we will have just 187mm (7 and a half inches) from the water surface to the top of the cavitation plate.

Look at any outboard motor instruction manual. The cavitation plate is supposed to be just an inch or so below the water line. With 7 inches you're way deeper than you need to be.

Fabbian

[Unicorn Dreams]

Welcome Aboard Cruisers Forum and enjoy the site...

[44'cruisingcat]

Originally Posted by fgd3

Look at any outboard motor instruction manual. The cavitation plate is supposed to be just an inch or so below the water line. With 7 inches you're way deeper than you need to be.

Fabbian

For a planing powerboat this is true, but for a displacement sailing boat it's not.

[44'cruisingcat]

Originally Posted by Polaris2

Hi from Adelaide, South Australia.
I have HitchHiker 37 catamaran. John Our boat has 2 pods under the bridgedeck that take Yamaha 9.9 high thrust motors. Its time to replace these and we are keen to use a model that has electric tilt, as it can be tricky manually raising and lowering motors under the deck. The clearance between the 'transoms' on our motor pods and water level is 350mm (14 inches). We currently have extra lomg shaft motors that put our props well down in the water. However the new electric tilt motors (Yamaha 9.9 GEPL) don't come in the extra long shaft. That means we will have just 187mm (7 and a half inches) from the water surface to the top of the cavitation plate. The pods are sited about one-third of the boat length from the stern, so the clearance is reasonably stable, especially compared with a stern mounted mnotor, but is 187mm enough? The Yamaha salesman doesn't seem to have a clue. Former used car dealer I suspect. But this is a $10K investement here...Anybody have any ideas?
May all your destinations lie downwind under a blue sky[IMG]file:///C:/Users/User/AppData/Local/Temp/msohtml1/01/clip_image001.gif[/IMG],
Polaris

Welcome to the forum! I don't know if they will fit, but if they do the Honda BF20 comes with an extra long- 28 inch - shaft with power tilt. A "powerthrust", bigger diameter lower pitch prop is available with these engines. They have bigger alternators too

My motor transoms are 360mm above the water, and the props are DEEP! Around 400mm to the prop centre.

Downside is it would be a $12k+ investment though.

Another option could be the new Tohatsu 20's which also have the option of an extra long shaft. MFS20C | Tohatsu Australia

[Cormorant]

I'm no expert, but it seems to me the key part of your situation is here: "the pods are sited about one-third of the boat length from the stern, so the clearance is reasonably stable . . ."

So the likelihood of the props coming out of the water in a seaway is much less than with a transom mounted prop. Seems to me that 7" to the top of the cavitation plate is fine in your case.

We have a transom mounted Nissan outboard with 25" shaft, prop center about 15" underwater (so cavitation plate at ~10"?), and even pushing to windward in big waves or chop, we don't get the prop spinning in air, though the transom frequently rises well above the water level. If that prop was at 1/3 hull length forward from our transom I can't imagine how we'd ever get it airborne. . . .

So is your question more about hydrodynamics, i.e., will you get full thrust with the prop relatively close to the hull bottom? Again, I'd guess you're fine, but others will surely know the nitty-gritty facts on this.

[GordMay]

As Fabbian said in post #3:
“… The outboard should be mounted high enough on the transom so that the exhaust relief hole will stay at least 1 in. (25.4 mm) above the water line when the engine is running at idle speed. This will prevent exhaust restriction …”
See: installing an Outboard to the Boat Transom from Mercury Outboard Rigging technical manual 3rd edition

[Polaris2]

Thanks to all for your comments and suggestions. AmyTom, I'll follow up again with local Yammy dealerrs to see if anythings changed in the last few months. fgd3 and Gord May, I think 44' cruising cat is right- there does seem to be a different set of requirements for planing boats and displacement hulls. The mercury rigging guide makes the same point, but unfortunately does not go on to give any guidance for displacement hulls. I suspect and hope that Cormorant is on the money. 187 mm of water over the plate should be enough... My concern is cavitation rather than performance, and I don't want to go to larger heavier motors for a range of reasons. So it looks like a bit more research before taking the plunge. Need to do it soon tho - can't get out of the marina safely on one motor. Funny thing about sailing boats - it always seems to be the non-sailing bits - motors , electricals, etc that cause the hassles. Sails are much more reliable. Thanks again to everyone for you help.
Polaris

[sandy daugherty]

Don't try to substitute any other brand for a boat designed for Yamahas. At nearly 3:1 gear ratios, the 9.9 high thrusts develop more thrust at catamaran speeds than the BF20; Honda's 2:1ish gear ratio and prop diameter limited by the power leg design never gets that high in their power range; my crude estimate places the 20 hp Honda at less than 9 hp at 8 knots. It was built to power light weight dinks at twice that speed. And Honda's outboards aren't as durable as Honda's other excellant products.

[44'cruisingcat]

Your crude estimate is wrong. My Hondas reach well into the recommended 5000-6000 range at full throttle. Boat speed around 9.5 knots.

What propellor did you base your estimate on?

The new Yamaha 9.9 isn't as good a motor for this purpose as the old one. It has dropped around 100cc in capacity. For some reason the power tilt isn't available with the long shaft. There is no standard pull starter.

The OP is obliged to install a different design of motor, even if he stays with Yamaha.

[capttman]

Originally Posted by fgd3

Look at any outboard motor instruction manual. The cavitation plate is supposed to be just an inch or so below the water line. With 7 inches you're way deeper than you need to be.

Fabbian

I think you might have meant the cavitation plate should be about an inch below the lowest point of the hull in the water

[Jimbo485]

What did you end up going with, Polaris?
We are in a similar position to you and are interested to see how you got on.

We have a Kelsall 40 with twin Yamaha 8 hp. Yes, just 8 hp each. One has been out of action from the Bahamas to the ABCs. Not a problem while on passage but there have been a couple of stressful moments in crowded bays. Even with both OBs working, we are underpowered in windy, enclosed situations, so we are considering upgrading to 15 or 20 hp each.

Feedback from your research or experience would be great, mate!

[Polaris2]

Well Jimbo, looks like we will end up getting the extra long shaft (25") high-thrust Yamaha FT9.9s. The low gearing combined with the big prop seem to provide the best option. Here in Australia for some reason we cannot get the extra long shaft with the power tilt. We also have the crazy situation of Yamah not providing a warranty unles the ("portable?") motors are installed by Yammy dealer - who have no idea of any installation thats not on the transom of a trailer fishing cruiser. And they want to charge Around $2k for us to tell them step by step how to do it... So, we found a place in the US that can provide the long shaft electric tilt. Have a look at
Yamaha Outboards 9.9 Horsepower
We have a fair bit of windage on our 37' cat and found the yammys were fine when both were running ok. Not much speed (max around 6.5kts) but adequate grunt.
Cheers,
Polaris

[Jimbo485]

Thanks, polaris!