Hi all,

For various reasons mainly my health hasn't been to good and I have been working on my boat as and when my health has allowed, I haven't as yet managed to get out for a sail. My boat is a MK3 with the outboard in a well, and I was wondering how much drag I would suffer if I left the outboard in the well when sailing? I can lift it out and lay it down in the well, but that is a bit of a pain really, it's a 6hp Mercury, and is a bit heavy to handle comfortably especially if the boat is dancing around a bit. Any comments would be appreciated.

Cheers, Ray.

Seal 22 MK3 "Morveren" Number 384

(the last one built by John Baker)

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I had a Merc 7.5 in a well on my Sinbad and more recently a Merc 5 in a well on my Parker 21. In both cases I considered it imprudent to try to lift it out when sailing - even when racing!

I did however notice that there was much less drag if I let the prop spin in neutral than if I stopped it by putting the gear in reverse. You can tell this by watching the way the motor kicks on its flexi mounts when you drop it into gear while sailing.

When I mentioned this in sailing circles I was always challenged - apparantly it's a known fact that a prop has more drag when spinning than when stopped. I made a rig to measure the true facts and indeed the drag at 4.6 knots was 4.7 Kg spinning and 11.7 Kg stopped. OK so 4.7 Kg slows you down a bit, so it would help to pull the motor out of the well, but what if you need it in a hurry???

Geoff Harwood ex S/287 ex P21/30

Many thanks Geoff, I am not in the least interested in racing, but I was just a tad concerned that leaving the engine in situ might slow the boat more than I would like. I feel that it will stay put now.

Cheers, Ray.

Seal 22 MK3 "Morveren" Number 384

(the last one built by John Baker)

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I always leave the motor in the well when sailing, but then i am not into racing. Undoubtedly it causes sme drag but not enough for me to be concerned about, compared to heaving it in and out without good reason. Moonspinner has a 5hp 2stroke Tohatsu which is adequate and weighs around 20kg. another advantage of leaving it in the well is that there is no chance of water getting where it should'nt, which could happen in a seaway when heeled with the power head on the leeward side (poining down below horizontal). This could be more of an issue on a 4 stroke, as there is a chance of the stern locker ending up with sump-oil slopping around, which would mke a real mess of your spare anchor warp? As for in gear or otherwise, I put it in neutral - ready for a quick restart if needed in a hurry - this is something you can't do if you have taken it out of the well...

Greg...

Seal 22 - Moonspinner

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Hi Greg, thanks for the comments, I must admit I hadn't given a thought to water or oil getting where it shouldn't, (not very bright above the neck see)[:D] I had come to the conclusion that I will leave the motor in situ, and put up with whatever drag it causes. I agree with you, that leaving it in neutral is probably the best option.

Cheers, Ray.

Seal 22 MK3 "Morveren" Number 384

(the last one built by John Baker)

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Geoff

The results of your empirical test comparing the drag of your outboard propellor in gear and in neutral really grabbed my attention.

As a lifelong reader of technical articles in power as well as sail media, I have never read of any such tests before.

Perhaps you have hit upon a key difference between inboard and outboard engine propellor characteristics when used in sailing boats?

On the other hand, perhaps the real justification for the common understanding that inboard engines should not be left in neutral when under way has a lot more more to do with minimising wear of shaft, shaft bearing and gearbox than reducing drag? [plus in many cases eliminating annoying that shaft rumble!]

Sounds like a great basis for an article in PBO!

Geoff Sheddick

Parker 27/146 "Stroller'

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You're right, the situation with an outboard is different to an inboard. But it isn't the design of the prop, it's the difference in friction in neutral. I found that my Merc needed 10g cm of torque to turn it whereas a 1GM10 (on a Catalac) needed 224g cm. Assuming the prop turns at 10 revs/sec in neutral (just a guess) the inboard is taking 0.213 hp out of the system compared to 0.009 for the outboard. I posted this on the PBO forum at the time and there was some discussion but it still comes up again from time to time.

Geoff Harwood ex S/287 ex P21/30

Geoff,

Very interesting information. Thanks for posting it. The forces you recorded are small and they agree with my own observations - though I've not made measurements. There is no doubt that the drag increases markedly when I put my outboard in gear: it's kicked aft, as you said.

I wonder how much of the total drag is caused by the hole in the hull rather than the motor. I've read tests of other boats that reported serious drag and speed reduction with the hull plug removed. I don't see why our boats should be different but that's not what I've seen. I did a back-to-back comparison, noting the speed before heaving to, wrestling the motor out, fitting the hull plug, resuming sailing and noting the speed again. I had to average the speed by eye each time as it varied a little in the usual way, of course, but I saw no noticable difference between before and after.

I did this test on a whim (after pondering while sailing single-handed!) in somewhat lumpy conditions that were far from ideal for the test - a smooth-water test would be better - but it persuaded me that I wasn't missing much by not wrestling with the engine more often. Also, I was planning to design a tilting engine mount to make fitting the hull plug easier but it's dropped in priority now.

Having said that, I would guess that the drag would be more significant in light airs - another test needed!

Anthony Russell

235/02 Sea Wyche

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