Rebuilding engines - what's involved, when to do it, how much will it cost

[oldharry]

Road diesel engines are reckoned to be good for 200,000 miles if looked after, and are subject to much more stress, constantly changing load. As a rule of thumb it's reckoned 2.5 hours marine use is equivalent to 100 miles on a road engine, so at 3250 hours your engines have plenty of life left. What does go more quickly, as others have said, is the ancillaries like heat exchangers, pumps, and electrics, because of the harsh environment.

You won't get any more power from a rebuild. Re-engining is the only way to go if that's what you are looking for. Full reconditioning should include a rebored block, new pistons and rings, regrind crankshaft, new bearings, camshaft, valves, recut valve seats, new valve seals plus all new ancillaries. So with labour a significant cost, not far off the price of new.

 

[Tranona]

We're getting nowhere near the hull speed & that's what we would like to improve on.

If that really is the case there is something seriously wrong. The hull speed is just over 7 knots and this needs approx 35hp. If your engine is indeed 47hp and is fitted with the correct propeller you should be able to achieve 8 knots (with the stern squatting!).

The starting point before deciding on changing the engine is to understand your existing set up. The key information about the engine is rated maximum hp and RPM, reduction ratio of the gearbox and size of propeller (diameter and pitch). Then carry out some timed runs in flat water at 200 rpm intervals from 1800 up to maximum, which I guess will be around 3000rpm and plot on a table.

If the propeller is correct and the engine is 47hp then your maximum should be close to 8 knots. If not something is wrong, and more than likely the propeller.

If you do decide to re-engine (a £10k+ decision) then you certainly don't need 50hp. The most appropriate engine would be a Beta 38 as in the boat below, or a Volvo D2 40. Either of these with the correct reduction ratio and propeller would give you a speed of 7.6knots and comfortable cruising of 6 knots.
mcyachts.co.uk/news/new-listing-1980-colvic-atlanta-31-thou-swell/

 

[Winston Collinge]

If that really is the case there is something seriously wrong. The hull speed is just over 7 knots and this needs approx 35hp. If your engine is indeed 47hp and is fitted with the correct propeller you should be able to achieve 8 knots (with the stern squatting!).

The starting point before deciding on changing the engine is to understand your existing set up. The key information about the engine is rated maximum hp and RPM, reduction ratio of the gearbox and size of propeller (diameter and pitch). Then carry out some timed runs in flat water at 200 rpm intervals from 1800 up to maximum, which I guess will be around 3000rpm and plot on a table.

If the propeller is correct and the engine is 47hp then your maximum should be close to 8 knots. If not something is wrong, and more than likely the propeller.

If you do decide to re-engine (a £10k+ decision) then you certainly don't need 50hp. The most appropriate engine would be a Beta 38 as in the boat below, or a Volvo D2 40. Either of these with the correct reduction ratio and propeller would give you a speed of 7.6knots and comfortable cruising of 6 knots.
mcyachts.co.uk/news/new-listing-1980-colvic-atlanta-31-thou-swell/

Thanks for your reply. The real problem is not what the engine can achieve, but noise levels in doing so. The Thornycroft T108 is a noisy engine compared to modern diesels and pushing it above 1500rpm means very high noise levels. I would like to be able to run at 1500/1600 rpm at a higher speed without having to have ear protection.

 

[Tranona]

Thanks for your reply. The real problem is not what the engine can achieve, but noise levels in doing so. The Thornycroft T108 is a noisy engine compared to modern diesels and pushing it above 1500rpm means very high noise levels. I would like to be able to run at 1500/1600 rpm at a higher speed without having to have ear protection.

Most modern engines are designed to run continuously at between 2-2500rpm and are not noisy, particularly if you take the opportunity to properly insulate the engine compartment while you are installing the engine. Just because an engine is running slowly does not mean it is necessarily quieter. Fitting a bigger engine such as a 50 and running more slowly is not a good idea. The engine should be sized so that with the correct propeller it will achieve around hull speed within 200rpm of rated maximum. You can achieve this with either a Beta 35 or 38 which are essentially the same engine, but the 35 is governed to produce 35hp at 2800rpm and the 38 is "tuned" to run up to 3600 and 38hp. The power curve for the latter shows 35hp at 3200. To cruise at 6 knots you need around 25hp which on a 35 is about 1800rpm and a 38 is 2100rpm. The 35 will just achieve hull speed of 7 knots whereas a 38 will raise that by about 0.4 knots.

I have just had a similar problem with the engine for my Golden Hind 31 - same waterline length as yours but nearly 2 tons lighter. It had been fitted with a Perkins 35hp which apart from being tired was overkill. The boat needs 26hp to get hull speed and the choice was between the Beta 25 and 30, both at 3600rpm. Chose the 30 which will achieve hull speed at 3200 - bit less than I would like but OK and cruise (5.5) at 2200, 300 lower than with a 25.

All such choices involve compromises - a trade off between ultimate speed and cruising rpm in this case. from my experience the difference between 1800 and 2100 for noise is minimal (and way quieter than the old BMC) but gets more noticeable when you get above 2500rpm. So your choice would be lower cruising rpm with the 35 but loss of reserve power for adverse conditions.

Hope this helps

 

[Winston Collinge]

Most modern engines are designed to run continuously at between 2-2500rpm and are not noisy, particularly if you take the opportunity to properly insulate the engine compartment while you are installing the engine. Just because an engine is running slowly does not mean it is necessarily quieter. Fitting a bigger engine such as a 50 and running more slowly is not a good idea. The engine should be sized so that with the correct propeller it will achieve around hull speed within 200rpm of rated maximum. You can achieve this with either a Beta 35 or 38 which are essentially the same engine, but the 35 is governed to produce 35hp at 2800rpm and the 38 is "tuned" to run up to 3600 and 38hp. The power curve for the latter shows 35hp at 3200. To cruise at 6 knots you need around 25hp which on a 35 is about 1800rpm and a 38 is 2100rpm. The 35 will just achieve hull speed of 7 knots whereas a 38 will raise that by about 0.4 knots.

I have just had a similar problem with the engine for my Golden Hind 31 - same waterline length as yours but nearly 2 tons lighter. It had been fitted with a Perkins 35hp which apart from being tired was overkill. The boat needs 26hp to get hull speed and the choice was between the Beta 25 and 30, both at 3600rpm. Chose the 30 which will achieve hull speed at 3200 - bit less than I would like but OK and cruise (5.5) at 2200, 300 lower than with a 25.

All such choices involve compromises - a trade off between ultimate speed and cruising rpm in this case. from my experience the difference between 1800 and 2100 for noise is minimal (and way quieter than the old BMC) but gets more noticeable when you get above 2500rpm. So your choice would be lower cruising rpm with the 35 but loss of reserve power for adverse conditions.

Hope this helps

Yes, very helpful in putting my thoughts together.

 

[Bilgediver]

Do you have space for sound deadening. I was once on a Nauticat 33 where the owner had added this and it was extremely effective and cost a lot less than a new engine.
Available from many sources for marine use including Amazon.

 

[Winston Collinge]

Do you have space for sound deadening. I was once on a Nauticat 33 where the owner had added this and it was extremely effective and cost a lot less than a new engine.
Available from many sources for marine use including Amazon.

Thanks for the reply. I've already fitted sound-deadening to the engine covers and that has worked remarkably well. However the engine compartment sides has all sorts of equipment attached which makes it difficult to cover.
I'm receiving quotes for new engines and now forming the impression that its not the time to buy. Engine blocks are still in short supply and prices looked to have increased by 30% over the past couple of years.

 

[madabouttheboat]

Just as important as choosing an engine for its reliability, build quality and spares and service availability, you really should think about where the service items are on the new engine and how accessible they will be once it’s in your boat. I worked on a boat recently that had had its original Volvo replaced with a new Vetus. Seemed like a nice engine, but boy were some of the important bits difficult to get to.

 

[Winston Collinge]

Just as important as choosing an engine for its reliability, build quality and spares and service availability, you really should think about where the service items are on the new engine and how accessible they will be once it’s in your boat. I worked on a boat recently that had had its original Volvo replaced with a new Vetus. Seemed like a nice engine, but boy were some of the important bits difficult to get to.

That's a very valid point. Trying to change an oil filter tucked away behind an engine mount and another pipe is extremely frustrating. At the moment I like the Nanni and the Beta engines for their layout. The Nanni looks to have the manual oil pump ,filter & impeller at the front of the engine and a belt you can touch to see if the tension is right. This directive of covering belt drives might be a bad idea.

 

[Tranona]

The Vetus is bad with too many bits including the water pump on the left hand side looking aft. Equally the Volvo has the filters in the same place. No problem for new boats like my Bavarias which had a large access door in the aft cabin to reach them (and all the fueling gear). This is of course a function of the base engine and the original use. The Volvo base is predominately used in small garden tractors with side access, the Mitsubishi for generators, again side access and the Kubotas in construction equipment with front access.

Yes, you have missed the "cheap engine era which ended with Covid - and will not come back. When I first started looking in mid 2021 they were giving engines away - well at least big discounts. By the time I ordered in April 2022 the price had gone up nearly £1k and it was not delivered until August. Likewise good used engines, particularly sub 30hp.