engine upgrade

[arthor]

greetings,
after much pondering over whether to build from scratch or fit out a hull, we have finally decided to by a cheap boat that needs some TLC.
It is a Seamaster 25 that has a shaft and Ford 1600 diesel (39hp) that is raw water cooled, having been run on inland waters. Once we have wet our feet on canals and rivers, it is our intention to venture out to esturaries and a bit of coastal. I have read stuf by Seamaster owners that say the range is pretty seaworthy. Apparently, the original design was considered suitable for this but I was thinking that it is a little underpowered. I have plenty of work to do before getting into an engine change but would like to know what is feasible further down the road.
I would like to upgrade to a fresh water cooled engine but with more power.
I have seen some good prices on engines 80-100 hp and was wondering if it is possible to have something like that installed. I would have thought that a modern diesel of say double (+??) the hp won't be much heavier and would probably be more efficient.
Does anyone have any experience of Seamasters and the feasibility of upping the power. I would look to have it professionally installed by the supplier with the intention of a suitable prop being fitted at the same time.

regards
arthor

[marshmat]

Hi Arthor,

Have you found the Seamaster owners' club yet? ( http://www.seamasterclub.co.uk/ )

I'm not particularly familiar with this boat, we don't see them around here. So I'm not entirely sure whether the hull is capable of some degree of planing operation. Given that the stock engine was only 39 hp, she might be a pure displacement hull, in which case a bigger engine would only be a waste of time and money. If you have any good photos of the underside of the hull, they might be helpful....

[arthor]

Thanks for the reply.

I have seen it listed as a semi displacement hull in some adverts but am certainly not taking that as gospel. I have visited the Seamaster website but I am not sure that they list 'my' 25 due to the dates from and to of manufacture which is several years before mine was made.
I have also noticed some comments about bilge keels so they rested upright in an estuary when the tide was out. That said, on the site there is a picture of one being craned and she doesn't seem to have bilge keels.
Your point on displacement hull and waste of moneyis taken and to be honest I hadn't thought of that, just assuming it was your classic medium/moderate 60s/70s style hull.
I will try to attach the pic and your input would be most valued. Also, I am having another look on the weekend and I hope to ask the owner.

regards
arthor

[CDK]

The boat in the picture has a very flat hull and should be able to plane. For that kind of operation, with much more forces acting on boat, a sufficient skin thickness is required. The weight of your boat can give an indication: it should be well over 2500 kg.
Twice the present engine output will not get your boat into plane though, you need at least 140 hp to achieve your goal.

[arthor]

Thanks CDK,
I was thinking that about the hull shape myself. When I was thinking about building from scratch, I looked at Hartley designs. Classic 60s/70s designs and there is no mention of displacement/semi displacement etc. All they mention was the vee. I would say that the hull shape on the Seamaster is moderate (very) vee. It seems to me that she would sit on the water rather than deep in it as in a displacement hull.
Correct me if I am wrong but there are many grey areas between Dis, semi- and planing. Does that mean that extra power will start to take her up on to the plane rather than be totally wasted? I don't want to plane at waterskiing speed I am really just happy getting up to the 15 knots region occasionally.
My main thought was that if I were coming up an estuary against a tide of say 5 knots, I wanted some extra (useable) power in reserve. So would, in your opinion, some extra horsepower be usable, even if it didn't get me up to full planing speed??

regards arthor

[CDK]

There is a long thread "DIY tunnel drives" on this forum. My 26 ft. boat has 2 VW diesel engines of 50 kW and cannot reach planing speed (yet). Originally it was powered by 3.0 ltr Mercruisers and could plane very well at speeds between 20 and 28 knots, with 22 kts being the best fuel/speed ratio.
With 3.5 tons, my boat is heavier than yours, so you might achieve planing below 20 kts, but not much below that.

Because the angle between the hull and the surface changes when speed increases, a lot of extra power is needed to bridge the gap between displacement and planing. If your goal is well below 20 knots, the boat will not plane at full throttle but behave like a "semi planing hull" with the bow considerably lifted but the stern still deep in the water. Good to combat a 5 knots tide, but bad for your wallet.

A typical diesel burns 250 gr. / hp/hr. At 5 kts. you will probably need no more than 10 hp and use 3 ltr of fuel, at full throttle you'll probably reach almost 10 kts. and need 12 ltr. With twice the horsepowers the consumption will double, but the speed will not be more than 13-14 kts.

Of course you could keep on cruising at hull speed and use the extra power only when you need it. That's why the throttle lever is a lever, not a switch!

[arthor]

thanks again CDK, really useful input.
We have sort of talked ourselves out of the Seamaster and have narrowed it down to a Fjord 24 with 130hp or an Invader 22 145 hp, both petrol sterndrives. On top of that, I still have this nagging feeling that I am selling out by abandoning my build plans. Still, a few grand to find out whether a boat is for us is better than 5 years of sweat and thousands of hidden pounds.

arthor