Speed, but little power

[rogerroger]

My First 27 will comfortably do 6.5 knots under motor (8 hp Yanmar YSM) on a flat sea with no wind.

However, against a bit of chop and a good breeze on the nose she really struggles to make any way at all (a force 5 on the nose and a 1 metre swell off Start Point last year at full revs the log read 0.0!)

Any ideas ? I spoke to the previous owner who replaced a 2 blade prop with the current 3 blade and said that improved things and also alluded to a problem with the clutch ... but I didn't really understand!



Roger Holden
www.first-magnitude.co.uk

 

[bedouin]

It sounds is if you may have the wrong sized prop - any ideas what you've got?

The best advice is to talk to a propellor expert - they know all the answers; I spoke to a very nice man from CJR propellors last week - extremely helpful.

The conclusion for 'Bedouin' is that I am getting excessive cavitation with my current prop, and I need a bigger prop with a finer pitch. My symptoms are similar to yours - a have some problems making headway into strong winds/seas (I can usually manage 3 knots), and increasing the revs from 1600 to 2200 makes very little difference to the speed.

 

[Roberto]

6.5kt seems a very good speed for the boat.

I think the proper way to increase rough weather speed from zero would be to increase the pitch of the propeller. I do not know what kind of propeller you have, but fixed blades pitch can also be moderately increased.

 

[simonjinks]

Reproping will make some difference, that is if the wrong prop is fitted at the moment. I think that you are getting to the windage versus power problem, try motorsailing - it's cheaper and more comfortable.

 

[rogerroger]

not sure what size it is - but I was very sad and took a picture of it when dired out in Lyme Regis. I'll bung it on my web site and perhaps you'd be kind enough to take a look.

Have you got a name and number for CJR props ? I'd like to get this sorted while she's ashore.

I agree about increasing revs - hardly seems to make a difference. In fact, when trying to motor round Start Point I was convinved the prop had fallen off as we were making literally no head way at all ! I had to lean of the transom and check it was still there!

Roger Holden
www.first-magnitude.co.uk

 

[rogerroger]

I agree about motor sailing - this is sometimes the only way I can make progress.

It's when the other half goes green and demands to be taken to the nearest port with no heeling over that it really becomes a problem!

Roger Holden
www.first-magnitude.co.uk

 

[rogerroger]

yeah - 6.5 knots is good, which is why I find it so strange that she slows down so quickly in any weather.

I've been recommend a prop expert so I'll talk to him - at least replies seem to concur that the prop is the most likely problem.



Roger Holden
www.first-magnitude.co.uk

 

[Twister_Ken]

Re-propping might help, but there are a couple of other factors at play, generally attributable to bounciness.

Firstly, because the boat is pitching in a seaway, the prop is at one moment fairly deep down, and the next it's almost at the surface. The degree of thrust being delivered is proportionate to the density of water the prop is operating in, so in this situation the thrust is fluctuating. Secondly, as the boat bounces, so the degree of hydrodynamic resistance fluctuates, as does the aerodynamic resistance. If the high thrust fluctuation gets in synch with the high resistance fluctaution, the result is very little headway. Only way to damp this out is a) to get a heavier, longer, less bouncy boat, or b) to hoist some sail so that the lugger is pressed into the water a bit and becomes less bouncy.

Had this amply demonstrated on a Dutch polder some years back, when we had very strong winds dead on the nose as we came to a lifting bridge. Because we couldn't tack through we dropped sail and motored at full chat. It took an age to clear the bridge, much to the disgust (no doubt) of the waiting car drivers. As soon as the waters broadened, we were able to raise a triple reefed main, and the boat was soon travelling at 5 kts, albeit 10 or 15 degrees off the wind. Boat was a Carter 33, not exactly a lightweight flyer..

 

[rogerroger]

might be something I'll have to live with then - I'm sure the previous owner would have taken some advice before changing to the current prop.

She is a light boat and doesn't sit very deep in the water so this would explain the lack of thrust in a bouncy sea. When I need to clear stuff off the prop I can actually reach it with my head just above water so that show how close to the surface it is.

The "P" bracket was replaced before I bought her ... could this have anything to do with it ?

Roger Holden
www.first-magnitude.co.uk

 

[bedouin]

Check out their website:

http://www.cjrprop.com

There are lots of other good prop people arround - the reason I chose them is that they have an excellent range of props to choose from, including a feathering prop and one of the best folding props on the market (I think I'll go for a folding prop). So they could give impartial advice on the merits of fixed vs folding vs feathering.

Next time you go to the boat, look on the prop and you should find two numbers - the diameter and the pitch - which are essential input into the discussion. You will also need to know what the ratio of the gearbox is, and what clearance you have to determine what size prop you can fit. In your case it looks as though you may be restricted in the diameter you can fit.

I should point out that I am by no expert - but I have spent a lot of time over the last month talking to experts trying to work out the best solution for Bedouin.

 

[HaraldS]

Can you reach your maximum RPM? Guess you can.
The behaviour you describe seems to suggest plenty, maybe too much pitch, so you get the great top speed, but not enough surface on the blades to support enough thrust.
I'm not an expert on props, but that would be my guess.
If you don't have enough space for more diameter, there are different blade forms and there are 4-bladers. But those have a lot of drag when sailing.
Let us know what the prop experts say.

 

[Trevor_swfyc]

The power to weight ration and drag resistance may give 6.5 knts in flat conditions. In high head wind and boat stopping swell with 8hp you may not have enough in reserve to overcome the increase in wind drag on the expossed hull and increased water drag as the boat dives into a wave. When I purchased my mirage 27 a friend strongly advised to make sure the engine was a twin 18hp. Comming back down the Thames two seasons ago we ran into a force 5/6 head wind, tide running with us, off Tilbury we had waves which stopped the boat dead but we were still doing 5 knots with the tide and hardly any boat control, wife not happy.
I think the size of your prop and engine would be matched don't they calculate this by computer nowdays. Anyway the prop is the first thing to check being the cheapest. Also I have for several years polished my prop which is very effective at keeping barnacles off, as any weed or growth on the prop kills its thrust.

Hope this helps
Trevor

 

[JimMcMillan]

I have the same problem in my Scampi.7 Knots in the flat to Zero in a short lumpy sea.The more banging on the sea the less speed.I have put it down to;light boat,doesn't carry her way.Lack of basic engine power 10HP.Lack of thrust i.e.I am restricted in Diam. of prop.Only Answer a bit of jib and talk to her nicely?or a large dram!

 

[Miker]

My boat is a First 260 which sounds similar to yours in weight and prop position. I can't say that I've noticed any significant drop in performance when motoring into a stiff breeze. My top speed is touching 7 knts with a 9 HP volvo ( the calibration setting on the log is, I think, a shade generous). I think that I would have had difficulties in getting up the Fleetwood channel against a 3 knt plus tide if I lost much performance when motoring into a strong headwind.
The prop is now a 2 blade Autoprop but I didn't notice any such problems even with the old fixed 2 blade. Not much help I'm afraid other than to agree that it doesn't sound quite right.
BTW I like your WEB page. The winter "to do" list is a useful check against my spreadsheet which seems to get longer each winter.

 

[Twister_Ken]

Can't believe the p-bracket is anything to do with it. If it were you woouldn't be getting all that flat water speed.

 

[Avocet]

I'm pretty certain that everything is as it should be. The only problem is that you have a light, beamy boat with a lot of windage. Does it have a spray hood, dodgers, danbuoy, and all the other stuff one generally finds on yachts?

We had an Evolution 22 with a Mariner 5.5 long shaft outboard. In flat water, no problems at all, but sailing out of Fleetwood, any kind of wave would stop it dead. This was partly due to the fact that the prop lost its "grip" on the water as it rose near the surface and partly because the boat had no inertia to carry it through a wave.

I guess your boat has a 24 -25' waterline length so the speed is about right for that engine power. I think getting a bigger or coarser prop won't necessarily help much.

 

[PaulJ]

I would agree with Harald, I think the chances are that you have too much pitch on the prop. With reduced pitch your speed in flat water will probably suffer a little but performance in a chop should be improved - it's a bit like changing down a gear when going uphill in a car. However there is no getting away from the fact that 8hp is fairly minimal power for a boat of that size, it is very much an "auxiliary" engine which is fine for getting in and out of tight spots but not really enough for serious motoring when that is what you want to do. My last boat was a 28ft Stag which came with a knackered 15hp Renault engine. I changed it because the engine had basically reached the end of it's life and couldn't be relied upon. I didn't really need more power but I replaced it with a 20hp Lister - the difference was amazing, top speed was almost the same but she held her way in a sea much better and handling/stopping/starting were much improved.

 

[rogerroger]

Now that's interesting - you're the first person to venture the opinion that the engine is quite probably under powered.

I looked at a Colvic Sailor 26 which had a 20 HP Bukh which seemed a bit more like it.

It is the original engine (1980) and although it's been reliable it needs to come out for an over haul which is expensive so maybe I should buy another ... how much do you reckon it would cost for say a 16 - 20 hp ?



Roger Holden
www.first-magnitude.co.uk

 

[bedouin]

Conventional wisdom is that the minimum engine power for an auxilliary yacht is 4-5HP per ton. Yours is only just beneath that, so that is not the only problem.

IF it were purely a matter of engine power then you would expect to experience black smoke from the exhaust and an inability to reach full revs under load. Since this is not the case the problem is almost certainly that the existing prop is not using your existing power efficiently.

As has been mentioned above, fitting a finer and/or bigger prop may reduce top-end speed in calm conditions. You should first size a prop to give you more power at slower speeds - if that would give an unacceptable decrease in top end speed then you should consider re-engining.

 

[HaraldS]

I would too agree that the 8 hp is on the low end, and I have made the experience in that past that with engines on the lower end of the range, like 36 hp in a Rival 41, we would reach hull speed with no problem, but would not go too well into a stronger wind and seas. A sistership of a friend had an 80 hp in the same boat and it was a huge difference in such cases, no stalling, just goes straight through the waves. What was also amazing, was that the more powerfull boat was a lot easier to go back wards with in a narrow harbour.

On my curent boat I have a fuel flow meter which is kind of proportional to the horse power delivered. When I go in flat water I reach my cruising speed of 7 knots at 2200 rpm and it burns about 2 liters an hour. When I went in 25 knots of wind and the according sea, I could almost keep that speed, but at 3200 rpm and 7 to 8 liters an hour. I think this does say something about the power needed to go into a strong seaway.

If you can still reach the specified maximum rpm under those conditions, it could mean that your engine and prop are delivering full power, but it's not enough. But it could also mean that the prop is cavitating due to the slow down and that it is overpitched for this condition.

If you cannot reach full rpm in these conditions, than it is very likely overpitched.