expected engine speed of a merlin 28

[conor dillon]

hi all,

I having recently purchased a bilge keel merlin, fitted with a 20hp bukh engine and a two bladed prop. I find her a little slow when motoring! There is no rev counter fitted so I am working off the engine sound. she can get up to 5knots but i feel the engine is under preasure at this speed. I would have experienced this same engine in larger although different sailing boats where a much greater speed was achieved. Could anyone with a simular boat set tell me the expected speed i should be getting. i will be drying her out in the near future to antifoul and i will check the prop then and try to get some dimentions.

i know a three bladed prop will slow down the sailing speed but I plan to travel the SW coast of Ireland and on occasion the need for making way, over rides the desire to sail! also the two kids asking "ARE WE THERE YET" an extra knot or two could save my sanity. is a prop change necessary or common?

any experience is greatly welcome.

conor d

[Vegable]

This could be a long reply!!! And before more knowledgable people jump down my throat I'm speaking theoretically and in simple terms here!
First I don't know the Merlin so I'm speaking in general terms.
The top speed of your boat is approximately 1.5 time the square root of your waterline length (LWL). Some boats are 1.6 times some are even 1.75 times but a useful average figure is 1.5 times. I've got a 31' Renown with a very convenient waterline length of 25'. So, 1.5 x 5 = 7.5 knots which is my top speed. A boat with a 16' waterline length would have a top speed of 6 knots, so I assume you are somewhere inbetween these 2 speeds. These figures don't include surfing down waves by the way. (I registered mine doing 10.9 knots once surfing down waves in a force 6).
The reason for this is that the bow wave lifts the front of the boat up and the stern starts to dig in. The faster you go, the higher the bow wave goes and the more you dig in until taking it to the extreem, your boat is going along vertically! A speed boat has the power and hull shape to ride over the bow wave and then runs "flat" on the water on what's called a plane. Sailing boats with deep keels can't do this.
Now, if on a calm day you look at the waves flowing along the side of your boat as you motor along you will notice the bow wave then 3 or 4 (or more) equally spaced waves running along the length of your hull. These are called riding waves. If there are say 4 waves running through to the stern then you are motoring at ¼ boats top speed. If there is 1 exactly mid way you are at ½ top speed. If the wave is single and its 80% of the waterline length from bow to stern then you are going 0.8 of your top speed, and progressively if it's meeting/passed the stern then you are at full speed.
So by looking at this "riding wave" as it's called and by rough approximation you can calculate your boat's speed (through the water) without instruments.
Alternatively you can use a GPS or a log!
Prop sizes are another ball game and if you look on the Yahoo site there is a "folder" in the folders section which has 2 very good XL spreadsheets called "Prop Calc" (I think) and you can feed it all sorts of numbers and it gives you very expensive answers.
We used to play a lot of I-Spy!
Re the rev counter, ALL diesel engines have a governor on them to stop them over-reving - even sportscar turbo diesels, so you won't be able to over-rev your own engine. I've sailed, I suppose I should say "motored", for at least 15 years without a rev counter. If the boat is overloaded, as opposed to the engine, the boat speed will of course slow down and this will be very noticable when sailing into wind and a head sea, but you won't be doing your engine any harm. The wind and sea will be stopping the boat movement, but it won't have much effect on the water being churned up by your propellor - it's just not going anywhere! And diesel engines much prefer running under load.
Hope this helps and answers your question.
Lastly, if sailing around the south west of Ireland, beware salmon nets. Make sure you have a good rope cutter infront of your prop. (And mask, flippers, snorkel and very sharp knife!)
Regards
Mike and Annabella (R129)

[sakean99]

She should take full revs without any problems - and that should generally be around 3-3,500rpm. If the prop is correct for your engine & hull, you should be squatting noticeably at full revs.

[conor dillon]

many thanks for your replies. mike working on your stats my max speed should be about 7.3. i'm along way off that at the moment. as i said i'll be drying her out later in the month and will chack out the prop.

anagin thanks for yoyr information, much appreciated.

conor

[Shaun]

Conor,

I too have a Merlin 28, it is a fin keel. I smooth water I can easly get 6-7 knots with two bladed prop. I have to watch the speed limit in Weymouth Harbour of 5 knots!
I think you need to check the engine mounts. When I purchased in Sept last year they were found to be on their last legs. I had them changed over the winter and the general noise and viberation was much reduced. Warning, in changing the bolts 3 of the 8 of the bolts broke. We had to then move the engine into the cabin in order to drill out the broken bolts and retap. Therefore I advise clean the rust of the heads of the bolts and soak in WD40 for 2-4 weeks to losen the bolts, if you can losen the bolts you do not have to move the engine out. Whilst you are working on the engine you may want is the state of the exhust elbow with the temperature sensor, and the cooling water "Y" pipe on the front of the engine by the water pump.

Best regards

Shaun