Nearly Grounded At Felixstowe Ferry- Can You Help?

[spurnclass]

Would very much appreciate an answer to the difficulty I encountered yesterday as described below.

This year, I have been forced to replace my BMW 8hp diesel engine with a Honda 10p outboard mounted off the stern with a outboard bracket.

The outboard bracket has "slots" which allow the outboard to be positioned in the water at various different heights. I have been having some difficulty deciding the best height to use. At first, I utilized the farthest "down" position. This meant that when running a very low speeds in the harbor the outboard seemed to be at at very good height out of the water. But I started noticing last week that when running at 5-6 knots at just under full throttle the outboard seemed to be very low in the water. At no time would the head be submerged but only around 2-3 inches of the shaft would be above the waterline, and the point on the shaft where the cooling water is dispelled was probably at or under the water line.

With this in mind, I decided on my trip yesterday to raise the outboard much higher out of the water. At very low speeds in the harbor a great deal of the shaft was exposed, but at cruising speed I was much happier with the position of the outboard in the water. At cruising speed you could now see the coolant water being dispelled and dropping down to the water. A least a foot of the shaft was exposed, but the exhaust still well below the water line. Everything was going fine until we reached the very end point of the River Deben, past the horse sand at Felixstowe Ferry. We decided to pick up a Buoy just before the final red buoy which marks the end of the river, to have a break and a spot of lunch before crossing the Bar. The tide was flowing very heavily against us and wind was gusting maybe 4-5 straight over the bow. I decided to slow down to drift against the tide onto the buoy. I reduced the throttle on the outboard accordingly. At this point I realized that I had reduced throttle far too much and we were drifting backwards with the tide far far too rapidly for my liking. We were being carried towards the Baudsey side river bank, the depth was rapidly falling away. Very unfortunately we were also heading straight for a moored yacht (with skipper and crew on board watching). My immediate reaction was to apply full throttle to punch back into the tide. But to my horror the outboards propeller was now clear of the water and was having absolutely no effect. Even at lower speeds the propeller still wasn’t getting under the waterline. With the Baudsey shore rapidly approaching (thankfully we missed the moored yacht by about 3 feet) I reacted by turning the boat through 180 degrees, leaned over and pushed the outboard down into very lowest position on the bracket. I was then able to get control of the boat once more, altered course to the horse sand buoy and made for a prompt return to Woodbridge with my tail very much between my legs! (I didn’t fancy going over the Bar after the incident just experienced.)

That’s the story, here are the questions.

1. With me positioning the outboard higher in the water, I experienced absolutely no problems at low speed or at full throttle all the way down from Woodbridge to Felixstowe. What I cant understand is what made the outboard behave the way it did when we arrived at felixstowe ferry. Is it related to the tide/Waves/Wind?

2. What recommendations could you give as to what is a sensible height to have the outboard out of the water at? Is there any rules of thumb etc to base it on?

Thanks as always for your exellent help.

James

<hr width=100% size=1>Anyone got any info on 'the spurn class' yachts, 23ft GRP built by marine plastics in 1970-ish?

 

[jamesjermain]

This is a perenial problem with outboards mounted on transom brackets on small yachts. The tendency of the stern to squat at high speed and the pitching moment in a seaway means the outboard can, at one moment, be burried up to the top of the shaft and the next thrashing like an eggwhisk in clean air. Any height you choose will be a compromise and you will have to select a position which offers the fewest problems.

From what you say, your first and last settings were probably the best options since you had full control at all times and the engine was not so deeply emersed that the head was in danger of drowning. The only problem here could be that increaed back pressure in the exhaust will reduce power and there is a greater chance that, when you turn the engine off, cooling of the cylinder(s) might cause water to be sucked back into it/them. In any case you should always raise the leg when you stop motoring so that cooling water drains out and the outboard is well above splash level and less likely to be 'pooped'.

I would recommend that yoiu continue experimenting with height, starting with the loweest setting and progressively raising it until you start to get airation of the propeller when the boat is pitching in a moderate sea. Drop it back down one notch and you should be about right.

Two other thoughts:

Is your engine a long-shaft model? If it is not, I would recommend swapping it for one that is, it will give you much greater flexibility.

Is there any chance of mounting it in a well further forward? If you plan on using the outboard as a permanent power plant and you will be keeping for boat for a number of years, it may even be worth getting an outboard well cut into a suitable locker in the cockpit. These have many advantages including moving the drive leg further forward where there is less pitching; protecting the engine from the elements; making the controls easier to reach from a normal steering position.


<hr width=100% size=1>JJ

 

[Col]

Sounds like you need a longshaft outboard . You could have it mouted in the high position so the power head is well clear of the water and you would still have the prop deep enough to cope with the conditions you met up with.

<hr width=100% size=1><A target="_blank" HREF=http://www.arweb.co.uk/argallery/colspics> Cols Picture Album</A>

 

[boatless]

What you are seeing in flatter water at speed is the stern wave raising the water level at the transom. Adjusting the engine height to suit this makes some sense, but the downside is what you experienced. In bigger waves you are going to find it difficult to get enough speed on to raise the wave, particularly head to the seas. If you were trying to get it going downwind you would probably have had more luck.

I would suggest trying it halfway between the two extremes and see how you get on with that. In big waves, again going upwind, you may still have a problem and need to drop it further. Only experiment will solve it.

As for a rule of thumb, as deep as possible without drowning it. Sorry, but I don't think that there is an exact answer.

<hr width=100% size=1>my opinion is complete rubbish, probably.

 

[spurnclass]

Thanks to all of you for the excellent tips. Seems like the long shafted option would definitely provide greater functionality. We are currently in the process of placing a deposit on a large (ish) ketch with a 42 hp diesel engine (relief!), so the idea of the well will probably not be worth us doing (although it does sound like the best technical solution). The interesting thing is that when discussing the outboard solution, I was advised not to go for the long shaft. I will have to speak to the person in question to ask why they recommended this!

Last season having previously run the current boat (2.5 ton 22 GRP by the way) using the inboard diesel, and being relatively new to sailing I always found the engine was a great "get out of jail" card. The problem I am now facing is whether I have the confidence in the outboard to perform under heavier coastal conditions. We're not looking to go too far this season, but we do certainly want to be doing a degree of coastal cruises up and down the Suffolk coast.

Am I being slightly naive to assume that I wont have a problem spec wise under my current set up of the 10hp outboard and the transom bracket on these potential coastal passages?

Thanks
James

<hr width=100% size=1>Anyone got any info on 'the spurn class' yachts, 23ft GRP built by marine plastics in 1970-ish?

 

[philwebb]

Re: Nearly Grounded At Felixstowe Ferry- Can You H

Hi Spurnclass, Some thoughts:is your outboard a longshaft or a shortshaft? maybe
you need a long shaft outboard if you cant get enough of the shaft in the water
without the water coming too close to the powerhead. You may be able to buy
parts to convert it.Otherwise it sounds as if your boat may raising at the bows
when you apply power , so that the engine goes down in the water, then you
are raising the engine to compensate for this then finding at low revs the bows
drop down, lifting the engine out of the water. What you could try is setting the
height as well as you can with the boat stationary, then using the tilt mechanism
on the engine to tilt it backwards ie propeller away from the boat. Doing this should
convert forward thrust into forward motion rather than lifting the bows of the boat
when you apply power.

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[graham]

As you were approaching the mooring did you have someone up on the bow ready with the boathook?

The transfer of weight from cockpit to foredeck would have a significant efect on the height of the stern out of the water,

Only suggestion I can offer is to use the lower position for manouvering then raise it for more speed and when crew are all in the cockpit.

10 hp is quite powerfull for your boat.You may find that full throttle is making her "squat" in the water.Try motoring at about threequarters of throttle it will reduce the"squat "effect and probably make very little difference to speed.

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[oldharry]

There are 'standard' heights anbove the waterline for outboards, which the vast majority comply to: Standard shaft engines should be mounted on a bracket, pad or transom, the top edge of which is 16 inches above the working waterline. Long shaft engines should mount 20 inches above the water. The difference doesnt sound much, but in practice it is quite a lot.

Going far outside these measurements can raise the skeg too high allowing the prop to suck in air and cavitate, and may reduce the cooling water intake. Running too deep increases the back pressure in the exhaust (i.e. the pressure required for the engine to blow the water out of theexhaust). This normally only matters on very low HP engines as the back pressure absorbs power and may lead to difficult or even non starting - the old Seagulls often would not start if they were too deeply immersed for example - one of the reasons why they had such a poor reliability record - works at home in the test tank - not on the boat!

On a small boat, trim can vary enough as the crew moves around to lift the prop clear of the water - just as it does as the boat goes over waves, lifting the engine clear. A long shaft engine will help a lot to reduce this irritating characteristic.

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