Outboard Discussions

[Bill]

The exhaust in the ob well is going to be a problem no matter what length shaft

The long shaft engine has the advantage of putting the prop lower into the water thus reducing the possibility of cavitation (prop spinning out of the water in a rolling sea). The short shaft engine is easier to manage getting into and out of the well.

[Theis]

I put in a Yamaha 8 long shaft. However, I put in a new motor mounting board that raises the motor about an inch or two above the edge of the motor well. As I recall the original motor mount is flush with the well. That being the case, I have a long shaft raised about two inches.

It seems to work fine without cavitation.

[Tom_Nelson]

Bill , Theis

Thanks .

I started with this question because I have been using a British Seagull with a long shaft, but have a 15 hp and a 9.9 hp outboards that I'm interested in using.

The outboard well on Thistle Dew has been modified at some point and I am wondering where the top of the transom was originally, and if cavitation is a problem or just an inconvenience with the short shafts.

Does any one know the average distance from the top of the transon to the water?

Tom

[Bill]

Are you looking for the distance from the deck at the backstay chainplate? A couple of thoughts: The distance will vary depending on the displacement of the individual boat (tools, beer, BBQ's, ets), and how it's lying on its lines (bow or stern heavy). Over the long weekend, I'll try to drop a line and then measure it. In the distant past, the racing fleet floated a straight edge under the bow and stern and dropped a plumb line to measure the distance. We were trying to determine displacement without actually weighing the boats. Maybe I can find those data . . .

[Theis]

Regarding cavitation, I think it is more than just an annoyance - more likely dangerous. Cavitation generally occurs when you are in significant seas and trying to get somewhere (to beat a storm, darkness, driving through seas, whatever). When it cavitates, you lose power - and under those circumstances, that is when you most need the power.

I also can't believe it is good for the motor to rev up and suddenly have a load put on it, again and again.

On the other hand, if you mostly do fair weather sailing, cavitation may just be an annoyance that occurs when you hit a motorboat wake.

You mentioned the distance from the top of the transom. Did you mean the distance from the top of the motor well mounting board to the water? I'll try to measure that for you this weekend.

[Brendan Watson]

I have used a short(15") shaft, 4hp., in flat water, that pushed my Commander just fine.
I have also been caught in a tide rip where standing waves had
me at a near stand still wth the boat hobby horsing and about two
feet of green water coming over the deck, I fired up my 15hp, long shaft,gained weigh,
and was able to bear off to get out of tha hole. I never cavitated the prop
once. Its safe to say that you will never cavitate with a long shaft (20"), never.
I bet my 4hp. would have burned up and possibly not gotten me through the rip
Tacking as I would have had to do would have lenthened the time I was taking solid water over the bow,
and as I had only the main up with little way would have been dicey at best. Go with a long-shaft if you anticipate
rough water.
Cheers, B.
Commander#215

[Bill]

Here are the results of our scientific investigation into the question of the distance from the top of the transom to the water. Using the University of Chicago School of Economics protocol (first assume a line . . .) the following results were obatined:

29-1/2 inches with me holding the line while sitting on the lazarette hatch.

31-1/2 inches without me on the hatch Took a couple of tries to get the lead line to just touch the water.

For purposes of comparison, we also checked the distance from the top of the ob motor well at the forward bulkhead (where you mount the engine):

14-1/2 inches with me there holding the line.

15-1/2 inches without me on the boat.

Protocol includes about 3 gal in the gas tank in the lazarett, ob on the main cabin sole, boom off the boat no water in the water tank, but anchor rhode and chain in the anchor locker.

Anyone get significantly different measurements?

[Tom_Nelson]

Thanks for all of the information. When I asked for the distance from the transom to the water I meant from where the motor hooks on inside the outboard well.

Thanks again to all.

Tom Nelson

[Theis]

I measured mine this weekend. Leaning over the motor to measure (200#), the top of the mounting board was 13" off the water. Keep in mind that my Ariel has a lot of weight (120# of lead and 70# of sand, plus furnishings (anchors, lines, etc. beer, etc.). My Yamaha is a long shaft

My recollection is that when I rebuilt Solsken, I determined the height of the mounting board (which is 1 1/2" higher than the original board in the well) by taking the lower edge of the boottop as the water line. Then I determined where the waterline on the motor was supposed to be. Working up from there told me how high the top of the mounting bracket/motor board was supposed to be.

[walberts]

Regarding the problem of exhaust smoke from the outboard motor in the lazarette: We wrapped the upper half of the lower unit with a rubber material forming a "skirt" like a mast boot that we then wrapped around the engine well. That kept the smoke out of the lazarette and allowed us to close it up and reduce the engine noise in the process. Before, the engine would choke out from the exhaust fumes if the laz was closed. Now, we can run the engine at crusing speed with the whole thing shut.

The rubber material was left over from a backyard pond project. It can be bought at any pond supply shop. We wrapped it with velcro straps that can be easily removed. The engine is free to turn for manouvering with the "skirt" in place.

[Theis]

My understanding is that there has to has to be a low pressure/no pressure escape path for the exhaust that comes out of the back of the outboard. In other words, if the ports are plugged, you can ruin the motor.

My problem I had earlier is that when I put a rubber "gasket" around the back of the motor and that lead exhaust down to the water out the bottom of the well, there was too much back pressure when the boat was under power and the stern/gasket dipped into the wake.

What I have done now (with my Yamaha) is to build an aluminum/fiberglass "cup" around the exhaust ports at the back of the engine, and connect a neoprene hose to carry those fumes out a small hole in the transom. After almost 900 miles of cruising this year with the motor operating at full blast about half the time (sailing on the big ponds here - Great Lakes - involves a lot of motoring and, when beating in particular, motor sailing. So, I recommend the same resolution to others.

[mrgnstrn]

so you have not had a problem with hot exhaust fumes and the rubber neoprene tubing? is it the same as the black fuel line you can get in (insert mega marine retailer name here)?
and is your tubing hooked up to a thru-hull on the transom? pretty high up, right?
i am starting to consider a similar system.
my current system is a pair of small copper tubings bent to point out one of the ventilation cowlings. not ideal because they sometimes don't reach and sometimes fall out, filling the laz. with fumes.
not ideal at all.

[ebb]

Since we are now in an era of new outboards
and modified ob wells -
perhaps a better determination of optimum prop depth
would be from the waterline on the Ariel
or Commander
to the center of the prop.

Thus, 'the center of my prop is 6" below the designed waterline on my Ariel
and it doesn't cavitate in a chinese chop.'

338's high thrust Yamaha 4/8
is 16" below the D,W,L,... and it doesn't cabitate neither,
but it's not in the water
yet.

[Theis]

The neoprene hose came from the auto parts store. The plastic through the transom fitting came from West (Its Mar... something), and the right angle barbed plastic hose fitting came from ACE.

I too was concerned about the heat before I did this. However, none of the components gets hot or even significantly warmer than the engine cover. Heat is not a problem. The only two problems I can envision are corrosion (the gasses are corrosive, I assume), electrolysis (that is why I used plastic), and a buildup of unburnt oil in the exhaust tube and fittings (I'll find out about this one when the boat is pulled for the season.)

Incidentally, I have probably burnt almost 100 gallons of gas this summer, so it has had a suitable trial period.

[mrgnstrn]

the only reason i ask is that my copper tubing gets quite hot after about 15 mintues of run time. This is trroublesome because my copper tubes have to be pointed out of the vent port, and it only does that when i bend it correctly to do so. of course the tubing seems to always need readjusting, and that's when i notice it getting hot. which means that maybe more of my exhaust goes through my "relief ports" than on some other outboards. hard to tell. i will check into the neoprene at NAPA here pretty soon.
-out-