What speed to expect.

Hi all newbie here, be gentle with me.
I have a princess 30ds and a bit disappointed with the speed of it, it is fitted with a pair of volvo TMD41A's (150hp each) on shafts but has had a bar keel added at some point for a drying mooring.
It currently does 16 knots max, both engines run up to the 3900rpm they are rated at and sound to be under good load, can hear the turbos whistle up, no vibrations from shafts at all so sure running gear is all sound but I was expecting nearer 20 knots from this setup.
How much effect would this keel be having? the mounts are moulded in a bit but still proud o0f the hull, is it worth me removing as it'll be rarely used if left on.
Photo of hull and keel setup at http://orthoticlocum.co.uk/barkeel.jpg

 

These engines are rated at 3800 rpm, if you measure 3900, they are not delivering maximum power. Unfortunately, you can not judge their loading "by ear". Best info comes from exhaust temperature and turbo pressure measurements. If you are running on "eco-diesel" or equivalent, the output is reduced by roughly 8 %, so check what fuel you have available. The injector pump may be adjusted for fuel quality if you know you will always use low energy fuel.

Next, in order to help, we need info on gear ratio (standard 1,93:1 or 2,63:1) you have plus propeller dia and pitch.

From your photo I can say that this keel is certainly stealing quite a number of your horses; the combination with overspeeding engines explains the major part of the speed loss.

 

Thanks for the reply.
Gearbox is MS4A but don't know the ratio or prop sizes, not had the boat long and I doubt it will be out of the water again till winter now for prop size, do these gearboxes have ratio marked on outside?
Manuals show engine max rpm as 3500 to 3900, what would be the best to run at?
Probably dumb questions to most but I'm new to faster boats, been slogging away at 7 knots for the last 10 years.

Edited to add: This boat is also used a lot on rivers at low speeds so that is also a consideration.

 

What caused you to anticipate 20 knots ? Without knowing gearbox ratio and prop size/pitch, you are left guessing what the situation might be. Unlikely you will gain much by removing the ironmongery unless you can go up in pitch, seeing you are already at max engines revs.

 

Mainly based on boat type and available power, the original sales stuff suggests up to 25knots with twin 130's and I don't think they were fitted with anything over twin 200's.
Sounds like a pitch change is in order then.

 

I wouldn't imagine that keel arrangement would be causing great losses at a miserable 16 knots. If it is running seriously below the expected result, things like excessive weight, or the engine/gearbox/prop match-up, would be more likely than the keel set-up. I'd be looking to see what gearbox ratios and props are fitted to sister vessels with the same engines, particularly ones that are getting the extra speed, before spending money.

 

Given your boat is about 9 tons, 300 Hp on a good day will get you to about 18. If the boat has picked up weight, as they all do with time and in concert with that extra drag, 16 seems pretty reasonable.

 

Weight should be around the 4.5-5.5 ton mark.

 

The optimum rpm range for power with this engine is 3600 to 3800 rpm, no more, no less. Most TMD 41's for leasure duty have the 1,93:1 ratio and as long as your boat has a clean planing bottom (in terms of hull proportions and weight distribution) and a displacement below about 8 ton, 20 knots are the norm. Best propeller would be 19x16 or 19x17; if Canal cruising is a common pleasure, use the 17" pitch.

The MS box should have a plate attached to its side with the info. For prop info, check with the builder.

 

This is what I have:

Length 9.27m (30'5")
Beam 3.35m (11'0")
Draft 0.91m (3'0")
Displacement 8,820kg (19,448.1lbs)
Headroom 1.88m (6'2")

 

Yes I think that keel could be slowing you down quite a bit at max power. You might not gain much more than a knot in max speed, but for the same speed you should save a considerable ammount in fuel and wear and tear on the engine. The savings might even be enough to pay for a new prop and shaft if you ever get around to doing some damage.

p.s. I am also curious about the well where the shaft leaves the hull. Can that be filled in or faired some?
I am not just thinking about drag, but also how the turbulence might effect the flow past the propellor.

 

As an extra question whats the opinion of anodes on the underside? I have 2 bolted to the bottom just inside the P brackets, would they make a noticeable difference to performance and are they worth moving at the same time as removing the keel bar? I was thinking of fitting to the top of the trim tabs obviously bonding appropriately.

 

I doubt it would have much effect, But if the thing really is 8 tons +, that keel set-up will add weight as well as some drag, and whatever weight has been added elsewhere all adds up.

 

This is the info from Princess's own website.
Princess 30DS

Year Started: 1980
Year Finished: 1989
Length Overall (Inc Pulpit): 31ft 6in / 9.59m
Length Overall (Ex Pulpit): 30ft 6in / 9.39m
Beam: 11ft 10in / 3.35m
Draft: 3ft 0in / 0.91m
Approx. Displacement: 4.1 tonnes
Height Above Waterline to top of GRP radar arch: 10ft 1in / 3.07m to top of light mast
Height Above Waterline to Top of Fixed Structure (Excluding radar arch and light mast): 8ft 8in / 2.64m
Usual Engines Fitted: Option 1 Twin Volvo 200hp diesels on shafts/sterndrives
Usual Engines Fitted: Option 2 Twin Volvo 130hp diesels on shafts/sterndrives
Usual Engines Fitted: Option 3 Twin Volvo AQ151 (146hp petrols on sterndrives)

 

Something wrong there, a Bertram 28 was somewhat heavier than that, maybe the weight is sans engines and drive line.