efficient 10m displacement powercat (build thread)

Absolutely, the point still stands that at lower loading, lower RPM is required for best economy. I am not suggesting that you can cruise at 20K at 1500rpm on this boat. Merely the lighter loading of large engines on a light boat may lower the RPM at which best economy occurs compared to the same engines on a heavy boat/or boat of higher hull resistance. I have no idea how gropers setup compares to other boats. Probably the engines loads are still quite high, given that my mates 22 foot plate boat has a 200hp on it and weighs much less than than half of gropers boat and can do 35k. He saves no fuel going slower than mid to high teens. But of course a short fat boat is going to plough at sub planing speeds. I think gropers boat should use a lot less when at trolling speed.

 

I havnt tried running on 1 engine yet. I'll do that soon. My original plan was for the 90hp Suzukis - this would have been ideal I think. It would have dropped another 40kg off each transom and the boat probably would have been dead level on her lines instead of about 20mm low at the stern. Crusing at 20kts is 4000rpm right now. 17kts is about 3500 rpm.

Richard - the boat is close to her crusing displacement. We had 4 people on board , 300 litres of fuel, the fridge and snorkeling gear, 2 anchors and all the mooring gear and fenders etc. 100 litres of water, and still have heaps sof tools and stuff on board on cass ot all went pear shaped.There is not much more weight to add from this point... maybe 200kg once all the food and supplies are on etc plus water maker and dive gear.

I think cruise rpm should be between 4000 and 4 500 rpm. So I need to increase by about 500 rpm from where we are now...

 

For sure you can back off further with a larger engine, for a similar speed, but whether it will save a lot of fuel is debatable. A good test would be running a planing boat at a trolling speed on a small "kicker" Vs a much larger main engine. It doesn't make a lot of difference in most cases, it still has to deliver the same thrust at that given speed.

 

Yeah the amount of power required will be the same. But it will depend where in the efficiency curve each engine is operated. Say you require a 50hp outboard to be run at full throttle to maintain cruising speed, I would guess a 70hp will use less fuel at the same speed as running flat out is not that fuel efficient on a petrol engine.

Also in the other direction, using a 200hp engine to push a boat at 5k when a 10hp would do the trick at 2/3 throttle. The 200hp will be not be as efficient at producing the required 6hp compared to a 10hp engine.

This can easily be verified in the real word. A cruising sailing cat uses much less fuel when run on one engine compared to being run on 2 even when at the same speeds (in flat water etc) even though the power required to do so is the same. Actually the power to maintain the same speed is HIGHER when on one engine thanks to the drag related to keeping the boat straight with thrust only from one side.

Fitting a small single cyl 2.5hp to a dingy in place of the 15hp when cruising to and from the dock at displacement speed uses much less fuel per week.

Its highly dependant on each situation.

 

If the smaller engine set-up will cruise without needing to go close to full-throttle, it will almost always beat a bigger engine for fuel economy. Full throttle is too thirsty in just about every engine.

 

Seems gropers setup is right in the middle somewhere and the economy could have gone either way with the smaller engines compared to what he has now. I guess the smaller ones may have used less at lower cruising speeds and the bigger ones less at higher cruising speeds. Higher cruising speeds is what always ends up happening in practice if the conditions allow it

 

Full throttle drastically shortens engine life as well, another reason to avoid it. It may turn out he can cruise on the one engine, which would have over-taxed a smaller engine. Depending on the desired cruise speed, of course. One of the attractions of this type of boat is there is more flexibility with cruise speeds. You wouldn't try and run a similarly powered planing hull at 12-14 knots, but this boat should do that happily.

 

If it stays a decent distance away from full throttle, it won't make much difference. I'll be interested to know what the cruise setting in groper's boat will be in various sea condition. There may be different cruise speeds suitable for different angles to the sea. One of the beauties of displacement cats is you can slow down without fuel efficiency suffering, as with a planing boat dropping off-plane.

 

Yeah trolling speeds on trailerable plate boats are just outside of hull speed, and with that planing shape hull with a huge engine on the back brings for worse mileage than going at 20k. A problem this boat will not have.

 

see below

 

indeed - this boat doesnt seem to generate much dynamic lift due to the rounded chines. So it doesnt really rise up onto a plane and you dont really feel a hump to climb through, it seems to be happy at any speed although the michlet modelling does show a pretty bad wave interferance drag at the typical hump speed of around 9-12kts as richard pointed out. So it cant be good running at this speed compared to 8kts - 2000rpm or up at 15kts or higher where the transom runs dry...

i took her out again this afternoon in pretty choppy conditions... shallow water, bit rough etc. not huge swells- we dont get them inside the reef here, its just a short period wave which gets pretty steep due to the shallow water close to shore... According to the weather report from seabreeze we had about a 4 foot swell of around 7.7s period, blowing 17-19kts SE... have no idea what the accuracy of the wave conditions are tho... wind seemed about right.

The boat was slamming a bit in head seas - not bridgedeck slamming - that hasnt happened yet, just the bottom of the forward half of the hulls smacking the water... was a bit noisy and uncomfortable when punching into it, didnt feel right going over 10kts and the boat appeared to be pitching at a resonant frequency with these waves - quite nasty really. All other points to the sea were otherwise very good, save beam onto it as all wide cats get that snappy roll motion - i was especially surprised how straight it tracked running with the sea. As the wave conditions changed height and period in a different location, i was able to "get up it" and punch through it avoiding the resonant frequency i had before, it seemed to like being at around 23kts where the pitching of the boat settled right down in this particular sea state - still quite noisy as the hulls slapped the wave crests but the motion was much more settled. No video as this was all on dusk and i doubt it would have came out well -too dark...

 

Interesting. A bit surprised to hear that it was noisy. What is it cored with in that forward area of the bottom ? Pitching is a bug-bear of power cats in head seas, naturally sailing cats don't have that to deal with. If you have the freedom to speed up to reduce pitching, that sounds like a handy option to have. Are your overall impressions in line with expectations ?

 

We have also found it is better to go fast in waves than slow down even though its counter intuitive. My Skoota has Veed hulls forward which prevent slamming. Otherwise your experience agrees with ours

Richard Woods

 

well so far we have done 3 trips and totalled 110 nautical miles, fuel figures are only rough as i have not measured the fuel quantity very accurately, but to the best of my reckoning, used around 200 litres thus far. Average speeds have been about 50% of the total distance at 20kts and the other 50% at 17kts... We still havnt changed the props yet, but should get a bit better once we do. We should be able to get it down closer to 1.5 litres per Nm for a speed just under 20kts i reckon... All of this distance has been carrying a minimum of 4 people, often 5 people including plenty of booze and food, 120 litres of water etc... so its not lightship condition... we overnighted with 5 people on board last trip, we all slept in full size beds with plenty of room and still had room for 1 more on the collapsible dinette table...

We managed to blood the boat with some fish too 5 lovely examples of coral trout and a couple of sport fish such as near on 50lb Giant travally - the biggest ive ever landed, and decent size barracuda aswell...

And lastly, weve also had to nurse some nasty hangovers aswell, last week has been a bit of a celebratory blur

 

Do you notice much change in trim angle underway ? Compared to the static trim, that is ?