Trailer cruiser revisited... as a trimaran

There certainly must be an increase in drag, with the "displaning" stern, compared to a simple canoe stern. The wetted surface must necessarily increase and, if there is enough pressure under the flat to generate some lift, there will certainly be chine eddies to contend with- just like a planing hull when the water isn't quite breaking away cleanly. However, I suspect (but cannot confirm) that in some of the boats that use this shape, the added drag of the transom flat is considerably smaller than the added drag you'd get from squatting. Not incidentally, this shape only seems to appear on cats that are relatively fast and relatively heavy for their length, ie. are not as long as would be optimal for their speed/weight. Going longer is the obvious solution, but if you really have to fit too much weight into too short a hull, something like this might work fairly well.

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Update on my little project.... resistance and powering calculations for the new hull indicate that, at 2500 kg displacement, 10 m/s (~20 kt, Fn = 1.13) requires 3.2 kN thrust (I estimate +- 0.2 kN uncertainty in this figure). That is to say, indicated power is 32 kW / 43 hp. The propulsive coefficient for a well-chosen outboard prop is likely going to be in the range 0.6-0.7, so 61-72 hp to reach 20 knots fully loaded.... more likely we will fit a 50, as the sprint speed will not be needed when in "workboat mode" and the power requirements decrease at lighter displacements.

A sedate 5 knots, as will often be the case on the canals, demands a mere 1.5 kW indicated (approx. 3-4 hp at the propshaft).... a nice efficient cruise speed will be around 12 knots using 10 kW indicated (~22 hp at the propshaft).

 

Matt

Without seeing the shaft line and where the prop actually is in relation to the hull etc, i would best aim for a PC of 0.5~0.55. Aiming for 0.6~0.7 from the start is very very optimistic.

You will also need to take into account (or at least be aware of) weight and surface growth and wear...hence the power requirement in some 5 years time would be shy by some 10~15%, without continual maintenance, at best.

 

Preliminary Bp-delta calculations, using a Honda BF50 as a representative motor, suggest a PC of 0.62 should be achievable with a stock 11.5" dia. prop on this boat, and that's with a rather conservative estimate of the advance velocity (Taylor Wf = 0.72). A "big foot" gearcase with steeper reduction, swinging a larger diameter prop, would likely do even better. I wouldn't call it overly optimistic to shoot for >0.6, although 0.7 is probably going to be an unreachable upper bound due to the diameter constraint of an outboard's aperture.

True for something that lives in salt water all the time.... this is a trailer boat used mainly in cool, fresh lakes and so fouling is not nearly so much of an issue. Most of the time we will be running at 1800-2000 kg displacement, which reduces the power requirements to 70-80% of what they are at full load (2500 kg).

 

Matt

maybe this is a definitions misunderstanding..????

When you refer to PC...are you talking about the efficiency of the prop (eta), from the line of efficiency on the prop charts, or are you referring to the overall propulsive efficiency, ie effective power/shaft power (EHP/SHP).

Your boat will slowly, over time gain weight, not matter what you do. But for other growths, in FW you will ahve much less, especially of cleaned regularly. But wear and tear will slowly degrade the overall efficiency too. These are not concerns, just things to be aware of.

We have just had an operator come back to us asking why the ferry we designed 10 years ago has somewhat different deadweight today than when delivered. Same reasons.

 

Outboard engines, at least in N. America, are supposed to be rated at the propeller shaft.... so apart from the drag of the gearcase/skeg, the propeller efficiency (eta) should account for virtually all of PC..... unless I'm reading the textbooks wrong....
Of course, for the present purposes there's no point in splitting hairs that finely, since "50 hp" on an outboard engine really means "Probably between 46 and 55 hp, most of the time". A 3% difference in PC means nothing when you have a 10% uncertainty in your actual engine rating.

 

Matt

There is a big difference between the efficiency of what one can get from a prop...reading the charts..and what the overall "total system efficiency" will be.

Ive scanned in a typical chat i use. The efficiencies shown, are for the prop in idea conditions. But these are used only for establishing the size and what is the 'best' or most suitable, to get the maximum performance, given the data to hand...ie delta(opt).

Once the prop is in the water, many other factors come into to play which affects the overall efficiency of the whole system.

This is why naval architects refer to PCs as the "power in versus power out", basically. In other words, it represents how efficient the whole "package" is, engine, shaft, gear box, prop, hull etc. Of course the overall efficiency is effected by water flow into the prop, shaft angle, etc etc....this is already assuming one has selected/designed a prop with the from the charts with delta(opt) at the maximum value.

So, going back to my original point. depending upon how the shaft line is, the location of the prop etc...your PC...ie what will eventually be used to push your boat through the water as required power, may be different to what you "theoretically" can obtain by the prop alone, in ideal conditions, ie no hull, open water etc etc etc.

So, when on sea trails, we use strain gauges to measure the power output. That gives us one part of the equation. Laying over the EHP from tank testing, we get the overall PC for the boat.

Does this clarify?

 

Mat
You will have to consider a high thrust prop for your application. Moving a 2.5t boat with 50HP is not in their typical range of application. For example Yamaha make a high thrust 14 X 11 prop (pontoon series) that will do a bit better than the smaller diameter standard range under the heavy loading. Even with this the prop will be in partial cavitation.

You would need 60HP on the prop to get your 10m/s with 3.2kN drag.

Rick

 

Yes, it does. I have already accounted for the wake fraction, the change in 'delta' for the prop not being in open water, etc. in the calculation above. Leaving all those factors out suggests eta ~ 0.7 to 0.72 for the prop alone in open water; with all those various losses considered, I calculate closer to 0.62 for the overall PC. So I think we are talking about the same thing.

Rick- yes, it does look like a "hi-thrust" outboard with a steeper than normal reduction is called for. Recall that 20 knots is not required at max. load, only for short sprints at partial load (Rt is closer to 2.6 kN at 2000 kg displacement, 20 knots, ie. about 80% of what is required to hit this speed fully loaded).

 

I shan't hijack this thread any further re the displaning cat hullform other than to say that it's originator, Tennant applied it succesfuly to a range of vessels - many of them with quite low D/L. It has been adopted by many in the recreational cat arena since. There's no doubt that whilst submerged, the flat transom would be les eficient than a canoe stern. However, at speed the transom is effectively operating in the planing mode.
Now - back to the topic at hand. I'd concur fully with AH. Aiming for an overall propulsive efficiency of anything over 60% is certainly optomistic. You may initially achieve a bit more, but the cost of slightly overpowering is far les than the cost of underpowering...

 

Generally speaking, this is true.
I must point out again, though, that in my case, the ability to hit 20 knots is a sprint speed for when we're relatively lightly loaded.... most of the time, we will be operating at 10-15 knots or less, weighing 1500 to 2000 kg. With a tonne of cargo on board (2500 kg disp), we won't need as much speed.
I would rather not spend extra money on a larger engine than necessary, since even a 50 is going to be run at half throttle most of the time.

 

I know you know this, but one of the advantages that 4-stroke outboards have - in particular over diesels - is that they are happiest when lightly loaded. Unfortunately, if you stick within the Honda stable, jumping up to anything bigger immediately incurrs a weight penalty. Beyond that I would think that going bigger only offers advantages...
Yes - we (I) understand that your top speed is a low priority. However, there's no point in having a design goal if you don't strive to achieve it. Ok, other factors may come into play that will force you to compromise - they invariably do - especially on a lower priority items. But to include an optomistic variable in your calculations and then justify its inclusion by saying that the result is of lower importance, seems like a somewhat illogical way of going about it to me....

 

I agree with you, Will, on most of that.

From the calculations I have done so far, though, I do not think a PC of a bit over 0.6 is overly optimistic. It's a figure I calculated using Bp-delta charts for several propeller patterns common on boats this size, along with fairly conservative estimates of the wake fraction, the loss in prop efficiency caused by being behind a hull and strut instead of open water, a resistance value from a calculation set up to err on the conservative side, etcetera.... the math has me fairly convinced that it's realistic.

Please feel free, though, to point me in the direction of a set of equations that you have found to yield more accurate PC estimates, should you suspect I'm off base with these numbers. Not trying to offend anyone- it's just that, as an engineer, I'm more easily convinced my own calculations are off if I can see more accurate ones, than if I'm just told they look a bit optimistic.

 

After I re-read my own post, I rather thought you might (quite reasonably) say that!
I know you're not one to pluck a number out of the air... I agree that something around 0.6 is going to be close, but I'm yet to come across an installation that has managed 0.7.
Again - not saying it can't be done... I just haven't seen one myself and I always like to base my numbers on existing craft, where possible.

 

Matt

One can only say with a higher degree of certainty, a such, with a drawing of the installation and its relative location/position to the underwater portion of the hull. This is where the "numbers" obtain their source, not just from charts. The location is the all important key factor and a visual image aids this immensely and is the only way to say for certain what overall PC to use..

 

Thanks, W and AH

See attached.... this is for a 14" dia. prop on a "big foot" style outboard gearcase.
I have a suspicion that the wake fraction I'm using (1-Wt = 1.11 - (0.6 Cb) for Taylor wake fraction Wt, which gives 1-Wt = 0.72) is rather on the conservative side for this boat... but the other formula I have here (1-Wt = 0.83 * V(knots)^0.047) suggests that 1-Wt is closer to 0.95. That, I think, would yield an optimistically high advance velocity, so I've been using the block coefficient formula even though it probably errs on the conservative side in this case.
I'm not stuck on Honda, they're just a typical mid-range engine in a suitable size for which spec sheets are handy.... outboards are (supposed to be) rated at the propeller shaft, so gear losses etc. are accounted for by a reduction in "rated" horsepower.

Most likely, it will come down to a choice of a 50 or 60 hp outboard, either 4-stroke or DI, as I don't think I can justify the added weight/cost of 70 hp when the peak power would be used so rarely. But some 50s and 60s share the same block and are the same weight, in which case the extra ten horses would be easier to justify. We'll see what the market has available when the time comes.

And there is a good chance I will be able to do full-speed, full-scale resistance measurements on the actual hull before buying an engine for it; computer simulations can only tell you so much.