power problem

[beau]

I agree hull speed for a displacement boat this length is 8.5 knots, but it shouldn't take 60 hp to get there in a 5 tonne boat 8 ft wide.
I must be developing a lot of drag somewhere because of the extra weight in the stern.

I am trying to do something different and so far i am not succeeding, OK.

At the moment (in boating design) the only way of going faster, is by using a Planeing hull at a minimum of 50hp per tonne or two(or three) narrow hulls (the 16-1 rule) which is used in commercial catamaran ferries but is not suitable for craft in the 40-50 ft range.
I built 5 prototypes developing this triangular hull with a very narrow angle of entry and a flat wide stern. It worked in the prototypes and I have to sort out why it doesn't work at the moment.
It shouldn't take 60 hp to get there in a 5 tonne boat 8 ft wide.?????
I had a 25 ft Catalina full keel boat 2.5 ton and with a 10 hp Honda we easily got a displacement speed 6-7 knots.

What causes a displacement boat at 40 ft lwl to only do 8.5 knots is the development of a bow wave at that speed which forces the bow of the boat to lift and the stern drops(going uphill)
Well my boat is NOT doing that. There is no sign of a bow wave at 8.5 knots.

I have a rudder made but it is not installed yet.

I appreciate any comments or advise because if I can get this right and be able to get 15 kots (say) out of a 40 ft hull with reasonably low power it will be a significant achievement for a cruising boat.

If I thought that 60 hp was needed to get 8.5 knots in a 40 ft narrow displacement boat I would accept the fact. However at this point I feel that there are many questions unanswered.

[waikikin]

Beau, I been watching the progress on your tri, looks like a cool concept, so far as I remember from some pretty limited study in NA(from Ultimo school of boatbuilding knowledge) when length : beam aproaches 6:1 wavemaking resistance & the old WL square root thing starts to go out the door at 8:1 things start to flow evan better at 10:1 things are starting to go really good & at 12:1 or finer your into racing tri/cat forms, I think that this maybe assumed some low wetted surface round bilged hulls, dunno how a hard chine & higher wetted surface boat affects that but low wave resistance is the goal in fine boats & I wouldve thought that with your boat at 5:1 & shallow you'd be getting close & better speed, its surprising me too! Maybe its the floats resistance holding you back? Or the transom dragging(maybe add a rubber? fairing flap or something) you've mentioned or some kinda vortices off the chine & maybe interacting against the floats. I remember there was a Len Hedges alu sailing boat of similar length/weight & slightly greater beam that I think got up around the 10 knots with twin 25hp two strokes in wells & the river boats of Duck Flats/Michael storer of similar hull form get close to 10 with a pair of yammy 9.9 4 strokes, I even used to get 6.7(in flat water) on a 10 meter(but.75wl beam) cat with a single yamaha 9.9. Maybe you could talk to M storer about it cos he's got some experience in the style. Sorry about all the maybees but I thunk you'd be going better too. All the best from Jeff.

[fastcat435]

Do not move to much weight as far forward as possible
Your battery cable will be very long and that loses power and increaes resistance
Try to move weight to your centre of gravity and maybe more than just the battery,s to level out your cat
It will be more comfortable and safer as a bonus.
On your 39 ft cat the centre of gravity is probably 6 ft aft of your mast location
This is where you want to bring your weight and the honda,s are powerfull enough
If you want your cat to go up from 8.5 to 9.5 knots you need at least 20 hp extra per side and that is not wise to do increasing your weight on the backside and increasing tankage at the same time to feed these hungry motors

[beau]

Thank you Jeff, for your input.

Years ago I had a 26 ft Telstar Trimaran which had a quite narrow center hull. We set it up with a 9.9 hp outboard and used to get 10 knots easy. I put a 25 hp outboard and got 15 knots. So I upgraded again and put a 35 hp outboard. However at this point I only increased the speed to 17 knots because the bow wave started to form abd we fell into the "hole".

This is when I realised that the squareroot of the waterline rule is not always right and I started to look for alternatives.

I have large trim tabs which are not operating at this time, I am going to play with them a bit.

The idea of moving the batteries forward was just to test the weight distribution theory, They were not going to connected.

It must be a "drag" problem of some sort, I will keep you all informed of my progress.

[44'cruisingcat]

What works on scale models doesn't always "scale up" as you would expect. For instance, it's easy to build a 1/10 scale propellor driven model aeroplane that will fly at 200mph. But you wont get a full size prop plane to go 2000 mph no matter how much power it has.

[northerncat]

buy some time on a computer set up for tank simulation and run your cad design through it to see what happens at your weight with the hp youve selected
sean

[beau]

They were not small scale models but between 12ft and 15ft long. however I do agree with your comments regarding scale.

It appears to be stern drag that is causing the problem. There are a number of changes I can make but first I will temporarily move weight forward so the stern is "clear" of the water, then I can decide whether I increase flotation in the stern or permantantly move weight forward.

[Evan]

The square stern is a clue - is it immersed at speed? If so, you're creating a lot of unnecessary drag.

[waikikin]

I,ve had a look back through the Beaus boat posts & recognised that I assumed the hull form was kinda more traditional, with rocker & thought the strait lines where a product of your drawing program(I still use pencils etc) & kinda skipped the Triflector innovation. I have always assumed as taught that low resistance hullforms were as much about cutting the water cleanly as well as letting it "join back" together sweetly at the stern. If getting the transom clear & releasing dont make the difference consider some of these maybees too. That as has been mentioned that a scaling issue exists in that with the smaller models the water flow could make the transition from the lateral attack angle of the triflector sides across quite a sharp chine to the bottom plating- with the scale up the areas & distances are much greater & an endplate/shearing effect occurs across the chine possibly resulting in a low pressure turbulent/even reversed flow zone under the bottom plating- in scows & barges having say a 300mm radius plate bent up for the corners results in much easier pushing & handling as water can more easily flow around the chines. Some testing in say a spot with clear water & anchored up in some swift current & use of some condys crystals solution & some cassette tape telltales on a wand or stuck with bagging tape might indicate the lines of flow but it'l be tough to get a speedi enough current as the phenomena might only become apparent at the wall/speed your getting now. Also on tug vessels that opperate kinda diffently to what your wanting at as in pushing slow but hard their nozzled props kinda drag water from all around in front & maybee(but I think unlikely) your out boards are doing a similar thing running behind the inclined but wide aft bottom plate trying to draw water kinda from all directions in front & again across & around that very sharp chine, mabee increase shaft length? Also a couple of years ago I had a good look over some lovely canting keel yachts under construction at Mconaghys factory- some of these boats have like a "wet box" that holds the actuating rams for the canting- into this box some air is pumped to evacuate the water(weight) & this I dunno if its true but maybee there's a perseption that the bubbles coming out assists in less drag aft maybee like the channels on surfboards can. Please dont think I'm having a shot- just kind of thinking "out loud" 'cos I really want your boat to go good too. All the best from Jeff.

[beau]

jeff,I really appreciate your comments. I will study them in a bit more detail and get back to you.
Enclosed are drawings

The red lines are the waterline

[beau]

the black waterline was the proposed and the red is what I ended up with when I put on more batteries and put them further back.

[Evan]

The immersed transom is killing you. It's probably costing you at least 2 knots, maybe more. Immersed transoms are best for planing hulls, but at slow speeds they suck (literally). A stern extension may be in your future if you can't shift enough weight.

[leehaefele]

If you were to change motors, you should get something that takes a bigger diameter prop. I have a Leopard 38 sailing cat that was anemic under power with 2X 42HP Diesels. I replaced engines (same HP but at lower RPM=bigger prop) and props (16" diameter) and now have power to motor against wind and seas. However full throttle in calm waters is still 8.2 kt and anything over 7.5 requires so much power and fuel that I never do it. Cruise is about 7kts on one engine at 1 gal/hr.

[Ex-Calif]

Beau - I posted in the other thread as well. If I recall your very first post you were looking for 20+knots with 60hp.

I agree with the other that the submerged transom is probably a killer. I would also investigate interaction between the bow displacement and the outriggers. I know you said that there is no wave but you must be displacing water laterally. This displaced water could be interfering with the outriggers.

Not a marine engineer - just shooting in the dark.

[OldYachtie]

Beau, you are trying to reinvent the wheel, and so far, it looks a lot like a cube. If you want to study naval architecture, go for it. It is not a simple subject, however. Surf on over to Boat Design Forums to begin to get a clue.