Speed, but little power

[Guest]

Hi Roger, Lots of good advice here, as usual. Judging from this and your other posting you are going to have to accept that you have a great boat that is underpowered for some of the things you ask of it. However all is not lost and I have been in the same position as you with an MD5B (7.5hp) in a Highlander 28.

To make better progress with you current set up motorsail, or try not to get into a position where you have to motor into the teeth of something nasty to get home. ie better passage planning with optional destinations given FM's limitations.

It is probable that the Yanmar is not actually kicking out 8hp if it is the original motor. The old PMX series yanmars could suffer from clutchslippage which was a simple adjustment. I doubt that yours has the same set up.

There is more than one solution to the power problem. I replaced the MD5B with a 2nd hand MD7A (13hp). Kept the original saildrive, and uprated the prop. Total cost £810 and I still have to sell the MD5B. Would I have replaced with a Volvo if I were buying new? No chance. Am I pleased with the result? You bet. Love the boat even more!

Last piece of advice.. don't rush into anything. Use the boat while you decide what to do. If you are going for a bigger engine, then fit a folding prop. Work out how much faster you will sail with one, then work out how much you would have to spend on a boat with that performance..'s easy really! You'll love the boat even more! ...and you might prefer sailing into those F5s than motoring!

Good luck,

KCA

 

[rogerroger]

Thanks for that.

I'm actually getting new sails in a few weeks so perhaps when motorsailing into a strong breeze I should get better performance and point higher.

I think maybe I've made it sound like I motor everywhere! but that certainly isn't the case - I make every effort even to sail onto my mooring.

It's more for the benefit of the other half. We set off from Dartmouth on route to Salcombe last summer, forecast was for F3/4 and slight to mod sea. (I'd quickly learnt not to take the Mrs out in anything higher so off we went).

By the time we got to Start Point there was a very large swell and a good F5 coming right from the direction we wanted to go. We'd been sailing great about about 5 knots but would have to tack all the way and her-indoors was hating it. So put the motor on to head straight to Salcombe and watched the log - 5 - 4.5 - 4 - 3 - 2 - 1 - 0 !!!

What made it worse was this ketch that just motored merily passed!

So turned round and went back to Dartmouth and by the time we arrived the wind had swung to the north so we crept in at about 2 knots even with a flood tide!

Where's a good place to look for a 2nd hand engine??

Roger Holden
www.first-magnitude.co.uk

 

[Roberto]

I would respectfully disagree with you about overpitched propeller.
If you can get max rpm from the engine then at best the propeller is correctly pitched, or else underpitched.

If you cannot get max rpm, and when asking max power the engine puts outs black smoke then the prop is most likely overpitched.

In the case described the prop would be at best correctly pitched for smooth water running but very likely underpitched for rough waters. At least increasing pitch would be the first, less costly remedy worth trying (maybe borrowing a different prop).

About prop pitch, you may want to check a meaningless thing I wrote at www.boatdiesel.com under Articles, from experience in quite a few boats.

 

[Guest]

Not suggesting for a second that you motor everywhere, Roger!! I got my engine through a club mate; I was able to see it in his boat and test it before he changed it.

There are a few places around with 'recon' engines, but try the classifieds. The going rate is about £500 upwards. As the green volvo plague is rife, it is likely that you will find a lot more of them than any others. Most Volvo dealers have second hand stuff, but it is a bit of a lottery.

As I mentioned, take your time...planning is everything. Make a list of engines you might consider for your boat, get the dimensions and make sure that they'll fit, that the exhaust will go where it's supposed to etc, and that the right prop will swing under your hull. Armed with that you can make out a shortlist and go hunting!

I occasionally hear about some. I'll let you know if anything comes up.

Cheers,

KCA

 

[rogerroger]

no black smoke

I try not to push it but some times get some white smoke if I do (I'm fairly sure it's smoke, not steam)

Roger Holden
www.first-magnitude.co.uk

 

[bedouin]

Re: no black smoke

I think Roberto is rather oversimplifying things - he is glossing over issues such as prop-slip and cavitation, which are in turn related to the diameter of the prop (and the DAR).

In a flat calm - if you were motoring at say 5.5 knots, what engine revs would you be using? What is your observed maximum revs?

 

[rogerroger]

Re: no black smoke

Hmm, don't have the luxury of a rev counter I'm afraid. Throttle is perhaps a little over half way to max and she feels perfectly comfortable.

Roger Holden
www.first-magnitude.co.uk

 

[Roberto]

Re: no black smoke

Even oversimplistic, but in practice how would you test/modify/correct for slip or cavitation... Among the many variables in this case, I would think pitch optimises the ratio between amount of work to be done and final result. Or, more correctly, preliminary result. I would definitely try to increase pitch before buying a new engine

 

[Guest]

I would agree with Paul here. I don't think you have enough overall power. I have a 32 footer with 20Hp and she'll do 7 Knts in flat calm but put her into head sea and we're down to 4knts. Brother has 27' with 10Hp and he can do 6.5 in calm but almost comes to stop in head sea. She could be described as a little bouncy boat. I would suspect that (if cost was not a problem) you could carry a Yanmar 20hp and solve all your problems.

 

[rogerroger]

Any idea what this would cost ?

Current engine is due a £700 overhaul so I'm questioning a new one anyway...

Roger Holden
www.first-magnitude.co.uk

 

[Twister_Ken]

Inland waterways mags

might also be worth looking in the back of a couple of canal boating mags - seems inland waterways dealers are not quite so rapacious as marine ones.

 

[bedouin]

Re: no black smoke

That's why you need an expert. There are programs that give an approximation to the behaviour with each type of prop - which is a good starting point.

If you have a prop with too great pitch and too little diameter you get a lot of slip and cavitation - and no progress (this is similar to spinning your wheels in a car). The fact that your engine can get to full revs is probably a sign that the prop is too small.

I can give you a simple example from my own boat. I can get 6 knots in dead calm on my 32ft 5 ton boat at about 1700 rpm. Since the engine will go to 2500 rpm this means that the pitch is too coarse - the engine can never get above 1700 without causing excessive slip. In heavy seas I have a similar problem to Roger, though not as extreme, in that I can wind the engine up to 2200 and still not make much progress - because all the energy is being wasted.

Just fitting a bigger prop would help because it would make the engine work harder at 1700 rpm; but the real solution is to fit a bigger, finer pitch prop. By doing this I will probably lose a bit in terms of economy in light conditions, but gain when trying to motorsail into heavy seas

 

[Roberto]

Re: no black smoke

I agree that increasing prop diameter is practically always the *first best* solution when there is a lack of performance.

However, in most if not all production boats the builder has already taken that into account. I mean that when they design a new boat, they measure the maximum propeller diameter available (from hull shape + clearance) which incidentally in 99% of cases will be theoretically too small, and then adjust to it a sort of convenient pitch.

Except for major builder mistakes, I am not sure that in a production boat you would be able to fit a propeller with a much larger diameter than the one that came with the boat.

 

[bedouin]

Re: no black smoke

That's just the problem I've got. There isn't enough space for the optimum diameter prop - so they've fitted a smaller prop, but of courser pitch, which is a very unsatisfactory solution for the reasons mentioned above.

I'm now faced with having to find a better compromise - which is why I am so full of information about props at the moment. In my case fitting either a folding or certain feathering props has the great advantage of moving the hub further down the shaft and so increasing the hull clearance (hence the size of prop I can fit).

All prop choices are a compromise - it happens that my priorities are not the same as the builder's and so I don't like the compromise he chose on my boat.

 

[rex_seadog]

I have to agree with several other replies - 8HP is simply on the low side for a 27 footer. Although I can't find the displacement on your web site I guess it's on the lighter side at about 5000lb, so in calm seas 6.5 knots is achievable. However, I can well imagine that with the light weight and high windage you could well be stopped in your track bashing into a F5 off Start Point.

On our Hunter Delta 25 we have an 8HP Mariner Sailmate outboard which is good in calm seas for 6-7 knots but with any sort of strong wind on the nose it's a different story - best to keep sailing. When we bought the boat it was fitted with a 10HP 4 stroke Yamaha and this had noticeably more grunt but was physically too big for the engine locker. Hunter recommend a 5HP motor which would be small enough to be retracted when sailing. This might be OK for the racing fraternity to get to the start line but a bit feeble for getting back to the pub before closing time with the wind agin us.

A friend of mine with a Dufour 2800 (perhaps a bit heavier than yours) has recently replaced his Volvo (13HP) with a 20 HP Beta at a cost of about £3000. He reckoned an equivalent Yanmar would have set him back about a third more. It's a very light compact engine and he seems pleased with its performance. Maybe he's gone a little over the top in HP as I understand you are supposed to work a diesel engine fairly hard. There was an article in PBO a few years ago on choosing the optimum engine size. If you haven't got it I can try to dig it out.

 

[rogerroger]

picture

I've bunged a pic of the prop on my website - not sure if it'll tell you much though.

click the "plans" link on the left then scroll down below the internal layout pic.

Roger Holden
www.first-magnitude.co.uk

 

[HaraldS]

Very nice site and a good article. As I said, I'm by no means a prop expert. All I can offer are observations I have made on many boats and some thoughts.

What led me to think of an overpitch possibility, was an observation that higher pitched props don't do too well in terms of delivering good thrust when you are not making much speed.

An extreme example of that was an experiment I made last summer, having our dinghi push the yachts bows from one side, and run the bow thruster against it. Just for fun. The thruster has 8 hp and the outboard engine 15 hp. Now guess who won? It was the thruster by a small margin.

The outboard engine has a prop that is specified to be optomized for 17 - 20 knots and I guess the thruster would be optimized for zero speed.

I also remember my old Seagull outbords, they were quite weak in terns of horse power but gave quite decent thrust with their strange prop.

All this led me to believe that pitch alone doesn't define the prop chracteristic fully and the active surface plays an increasing role when thrust is needed.

When we did the thruster versus outboard contest, by listening to it I think the outboard engine didn't quite reach full rpm, but came close, which suggest that at least the engine thought it was working very hard, (like the guy at Compass), but certainly the result wasn't what you would expect.

So my conclusion was, that under such circumstances not all power is turned into forward thrust but may be burned in trubulances around the prop.

I'm very curious what Rogers real problem is: Can his engine turn its 8 hp into forward thrust and does it just not have enough power, or does it waste a lot of energy in adverse conditions, and if so what kind of prop would help.

 

[HaraldS]

Re: picture

Roger, looked at your picture. My observations would be: (1) There is some clearance to go up a notch in diameter. (2) Not easy to tell from the side, but the blades look almost as big (wide) at the hub as the do way further out. If that's the case than the surface isn't really as large as it could (should) be. Pitch seems quite high.

 

[bedouin]

Re: picture

The standard formula is that props require a clearance from the nearest part of the hull of 15% of their diameter (= roughly one third radius). Your prop looks to be about that size so there is no scope for fitting a larger diameter fixed prop in that position.

As far as one can tell from the picture the prop is not particularly coarsely pitched and has a fairly high DAR. That suggests to me that your current prop may be near-optimal given your constraints. However don't take my word for it - get the figures and then either pass them to an expert or feed them into one of the prop-calc spreadsheets out there (do a Google search).

Note also - if that is the case then fitting a more powerful engine may not do much good - without fitting a bigger prop you would might not be able to use the extra power.

Being in a similar situation with the prop (though with plenty of power in the engine). I am looking at a folding or feathering prop. The folding props are no more efficient than the fixed, but they have the advantage that they mount further down the shaft (up to about 6 inches) which would give you more clearance - probably enabling you to go up at least one size. They would also dramatically improve your sailing performance. Try the Gori or CJR for good folding props.

There is a thread about the Autoprop elsewhere on this board - that might also be a good choice. A lot more expensive but undoubtedly the most efficient prop on the market for a sailing boat - its 'self-pitching' capabilities means that it adopts the most efficient pitch depending on the conditions, and is said to be particularly good for motor-sailing. It would also decrease the drag under sail and boost sailing performance. I don't know how much the Autoprop is offset from the current propellor position, so I can't say whether you'll be able to get one a size larger.

Once again - my standard disclaimer. I am not a prop expert, but I am going through similar issues so I can pass on what the experts have told me. I would recommend contacting bruntons directly about the autoprop, and cjr or gori for folding (cjr also do a feathering prop).

 

[Stemar]

I'm no expert on props, nor even a particularly experienced sailor, but from what I've seen, I'd have to come to the conclusion that your boat is underpowered.

I sail on a 26' Westerly bilge keeler. She has a 25HP engine, though, considering its age, I doubt we see more than 20HP from it. She's not the fastest boat around on flat water, but a head wind and rough sea don't seem to bother her. Coming round Selsey Bill into a F8 with wind over tide was quite ...interesting, but didn't seem to affect our speed all that much. We did need a few more revs, but still had some in reserve.

Our boat weighs in at around 7700lbs, which is probably quite a bit heavier than yours. The extra momentum does help, but windage is what keeps you from getting moving again when a wave stops you, and to overcome that, you just need more grunt.