Semi-displacement vs. Planing properties?

[Spi D]

Looking to buy my 8th boat since 1968, this time in the 30-35' segment of semi-displacement, diesel & shaft driven GRP boats, possibly with flybridge and/or aft cabin.
I've looked at French, Spanish, Scandinavian, American and UK vessels, from mid 80's and onwards because condition means more than age. The idea of semi-diplacement is to get a comfortable ride in "less-than-friendly-weather' without being limited to displacement speeds. I've realized that going fast in a boat of 30'+ and x tonnes isn't going to fit my fuel budget either

Weight of these boats vary but tend to be some 5-8 tonnes with good balance due to centrally placed engines.

From previous, lighter, boats I know that planing hulls prefer to go either around hull-length speed or fast enough to maintain a stable planing (same speeds that delivers best mgp), that a leading deep V design improves comfort in choppy seas and that wide chines tends to give some slamming in head seas.


Not all the boats on offer are true semi-displacement but rather planing, modified V-hulls and when looking closer at boats that are praised for seakeeping, it appears that very different designs are able achieve this coveted rating.

Can anybody explain, from experience, if heavier planing boats are closer to the semi-displacement behaviour in the mid-speed range?

Any comments highly appreciated

 

[duncan]

I got lost in there somewhere - what's the question?

There is a huge range of hull forms that are either designed to operate in the SD mode, or operate in that area because of the installed power source.

SD is the ability to exceed 'hull speed' without planning, but of course some element of the latter has to be there unless you are at the extremes of 'thin and long' or a multihull.

Boats like the Aquastar/Nelson are very different in hull design to, for example an Offshore 105.

Both are normally set up to operate in the SD arena, but put enough power into the latter and it will plane, whilst the Nelson isn't going to 'cheat' anymore than it's design.

Basically any planing boat with enough power can operate in SD mode, and the heavier it is the more 'planted in the water' it will seem - many many shaft driven flybridge boats (P35 would be an example in the same length range as the above pair) will act in a similar manner at the same speed going into a sea. However, all planning hulls have hydrodynamic lift in the stern, whatever mode they are operating in, and it's this that will affect their seakeeping with a following sea such that it's very different to a pure SD design.

If you look at the hull designs of lifeboats over the last 40 years you will see how designers have continually tried to get the best of both worlds.

 

[MapisM]

The idea of semi-diplacement is to get a comfortable ride in "less-than-friendly-weather' without being limited to displacement speeds. I've realized that going fast in a boat of 30'+ and x tonnes isn't going to fit my fuel budget either

Your idea makes sense.
But if the reason why you don't want the D speed limitation is that you're thinking of using the boat mostly at 15+ kts, I'm afraid you'll soon discover that a SD boat will not fit your fuel budget any better than a P boat.
In fact, the faster you want to go, the better will be the mileage with a P boat.
Otoh, if you wish to use the boat mostly at D speed and/or in rough sea, but just don't want to be limited for the occasional need to beat the weather, make it in time for a lock or tide, etc., then SD is perfect.

But at risk of making a sweeping generalization, in the 30'-35' segment (actually up to 40' or so), P hulls make more sense than anything else, imho.

 

[Spi D]

Thanks, both.

I'm all with you regarding the speeds that the hulls are designed for.

My consideration is if a planing hull of this size/weight will be as unwilling to go mid-speeds as my lighter planing boats were. They just raised the bow, tried to balance on the rear of the hull and used a lot of fuel.

On plane everyting was fine - only that the ride got quite uncomfortable, bouncing around from wavetop top wavetop.

If those bigger P boats will however behave under mid-speeds I'll keep them in my search.

BTW just viewed a P35 in Barcelona. A 1986 in good shape (the owner said). No damages to it so not defect as such, but 25 years of Med sun, only lifted every other year for antifouling and never seen a polish/waxing... Go figure..

 

[duncan]

Should be cheap yet come up fantastically with a thorough cut, polish and wax!

 

[Spi D]

Should be cheap yet come up fantastically with a thorough cut, polish and wax!

Yeah. Adding things up made it rather expensive and both electronics and engines/driveline were in need of attention. 2x Volvo 200 hp dating back to 1986, never opened etc. ran OK but filled the marina with white smoke and didn't run smooth. Can't say what they needed, but instruments indicated uneven oil pressures. Interior was in order but pretty worn etc. etc.

 

[MapisM]

BTW just viewed a P35 in Barcelona

If that boat ticks your boxes, you should pm hlb of this parish.
He's looking for someone who will continue to take care of his one as he always did, I reckon.

 

[Spi D]

Thanks, PM sent.

 

[piratos]

Planing vs Semiplaning

My first boat was 12', then 15/17/24/26/31 and now 40. The last two SD, and first 6 boats full planning. The world of SD is not so much different from a full planning boat. It will cruise at 15-16 knots in stead of 20-25 knots, but the fuel consumption is not so different as you my expect. Some SD will be happy with smaller engines than eqv planning boats and as such cheaper to built, but most of the SD built in the late 80'es are heavy, and need the power (if you want to use the speed potential).

The advantage of the SD is that it starts 'planning' as soon as it leaves the displacement speed, and often behave same way with 12 knots as with 16 knots. Most full planning hull as you say yourself are not good to run between displacement and planning speed. The SD is however happy to run exactly in this sector. In some weather conditions it will become nescessary to slow down to displacement speeds - and that goes for the SD as well as planning. You will find seaconditions that suit the planning better than the SD and other way round as well.

Personally I am happy with the SD - it gives me enough speed when I want a 'fast' cruise, but most of the time I am happy with the displacement speed.

There was earlier a thread about fuel consumption Displacementspeeds vs Planing. Also the SD is cheaper to run in Displacement speeds, but all of us tend to run at speeds that is actually beyond the real displacement speeds. If you run a 40' boat at 9 knots you are already too fast to be running really economic displacement speed (say 12 m loa / 10 m lwl = 7,7 knots)


The SD usually usually has some keel which the planning does not have. This way they steering should be improved in both displacement- as full planningspeeds.

Im sure that in this Forum you will find more favourizing the full planning over the SD. I do not - and if you asked for opinions you have mine

Good Luck