PowerCat Performance Calculator

Hi - I am new to this forum so please be tollerant if this has come up before!
I own a 28' Ron Given designed powercat. I purchased the boat second-hand and believe it to be grossly under powered with just 2 x normally aspirated 50hp Ford diesels. If COMPLETELY clean with a downhill sea it will peak at 14kt but after a couple of weeks on the marina berth that drops so she struggles to pull 10kt and just won't climb out of the hole.
I am now looking to re-engine and would like to access a calculator of some sort to give me a guideline of what I can expect from different horsepower units. My aim is to cruise at around 16kt and still have something in reserve - say 20kt max - but also to maintain the economy of running small diesels.
Can anyone guide me in this please?
Many thanks in anticipation! Cheers, Alan

 

Hi Alan,

(1) Welcome aboard bd.net

(2) I'm trying to figure out just what your boat is. Ron Given Design currently has 12 powercats listed on their website http://www.givencats.com/ and none of them are 8.5 m / 28 ft LOA. The closest match is the 8.2 m, 2.98 tonne Tamara-class. Can you offer more details, or photos?

(3) 14 knots on 100 hp is not bad for a boat that size. A good antifouling paint on the hull bottoms will do wonders for the speed loss you see after sitting at the dock for a few weeks- perhaps your current antifouling is worn out, or not meant for your climate?

(4) Repowering with substantially bigger engines sometimes works, sometimes doesn't. It depends to a large degree on the hull form, and the cruise speed for which the design was originally optimized. Something that's designed to be efficient at 10 knots will often have a very hard time reaching 20, if it can do it at all. Since most resistance prediction methods only work for particular hull types in particular speed ranges, it's important to know what kind of hull you have before anyone will be able to offer much meaningful advice. Some photos, especially photos of the boat out of water, would help a lot if you have them.

 

Many thanks Matt!

She is of the Tamara class (which as you can see was meant to have twin 100hp - 150hp outboards) so with just 100hp total she struggles a bit!

I am arranging for her to come out in the next couple of weeks and have full antifoul strip and repaint so I hope that will make some difference but doubt it will get me quite where I'd like to be.

I'm also looking at the possibility of turbos on the motors and re-pitching the props as I doubt turbos would give me more than 20% improvement and the gearboxes are rated for 60hp. I believe the shaft might be the limitter there as they are only 1".

Another approach is to extend the hulls by about 2 - 3ft. The real advantage of that would be to "bridge" the wavepitch of the Hauraki Gulf where we do all our boating although it should also keep her nose down under power and increase boatspeed. That won't happen (if at all) until winter which for us is a long way off - I hope!!!

I've tried to attach photos but keep getting error messages - tried zipping too but that didn't work out either

 

Reference Photos

HOPEFULLY I've managed to upload a zip of some reference photos now

 

nice boat that should plane better with some bigger horses and props

 

Hello,


Ron has done a fine job with his hull development and powering over the years. Sometimes the owners will request smaller engines for a hull that was designed for greater speeds. Without knowing the history, could be a possibility in this case.

I would recommend for you to approach Ron directly to ask for his thoughts on what to re-power the vessel with first. He would be able to give you his expert opinion on engine power required, and what size engine will fit in the hulls.

Apart from that, the hull appear to be full planing hulls. They were designed to handle more speed and HP.

I have found from my own experience with this type of hulls, that the cat requires ( by rule of thumb) about 100hp per ton to achieve a speed of 30 knots. The hulls are un-efficient at lower speed due to the wetted surface area of the planing hulls. As soon as the hulls are pushed with enough HP to get them over that 10-16 knot hump, they get on a plane and start to operate correctly.

Re-powering a vessel with larger engines may prove to be difficult and costly in some cases. The hulls of a cat are pretty tight at the best of times, so look closely at engine dimensions, and how to service them. Shaft sizes, and props will need to be increased in some cases. If so, check the prop size for the new engine first, as you may not have enough prop clearance.

A quick note: Cats are very weight sensitive. Ron's design is very well balanced by the photos you have left. I would not recommend adding any hull length to the hulls to over come your current problem. More buoyancy aft will push the nose down. In a following sea you may find the hulls either bury themselves or want to steer themselves. The further the nose is pushed into the water, the more they act like a brake, wanting to slow you down. So your viscose circle begins.

I would start with Turbos on current engines, prop size, and removing weight from the boat.

All the best,

 

Many thanks for all that advice East Cape - I'm already a fan of your designs.

I spoke to Ron a year or so back when I bought the boat and he was quite keen on the idea of extensions. One thought I had in doing that would be to allow me to shift the rudders and props aft (with longer shafts) and thereby increase the prop diameter - assuming I could get more HP.

Ron is a hard man to track down in NZ these days though; he spends more and more time up in New Caledonia and the islands - and full marks to him for that! I will definately try again to call him again though as he's a mine of info.

When clean she seems to plane at around 10kt and today I've booked her in for a bum-strip and new antifoul - at the same time the engineers can refit the stbd shaft that decided to ruin my weekend by migrating from the gearbox coupling on Saturday . . . . . . . ARGHHH!!!

Regarding turbos - the existing engines are based on the Ford Escort van motor (Chris Moon supplied) and the two deisel gurus I talked to yesterday both reckon fitting turbos will result in sudden and catastrophic outcomes! The 1" shafts appear to be 316 SS so they certainly won't take a lot more power. I believe I can swap these out for a stronger material 2205(?) but unless that will accommodate at least twice the HP I may as well up the diameter anyway. The existing props were cropped and cupped by Briskis just before I bought her so there should be some scope there.

For engines I've begun by looking at the FNM ATM100 or 120 and the Lombardini units 87 or 160hp. These seem to be the lightest in class but the cost is making my eyes water already . . . . . Apparently the 87hp units should return around 19kt (about where I'd like to be) and the 160s 30kt but with all the ancilliary cost - tankage, breathing, exhaust etc.

Adding lightness is a priority - the Plastic Mac 270 is going in favour of a new alloy RIB and most importantly; I plan to refit the interior (galley & dinette) using composites rather than the 1/2" marine ply and buscuit-board currently installed. Now, if only I can lose about 50kg off the waistline she should fly!

 

Alan
There has been plenty of good advice already but if you want to take a more detailed look yourself you might find this site of interest:
http://illustrations.marin.ntnu.no/hydrodynamics/resistance/planing/index.html
Just put in the dimensions for one existing hull. You do not need to get too concerned about numbers for the eps,a or f. THere is some interaction between hulls but I do not think it will be significant in your speed range.

If your weight of 3 tonne is right I get a thrust of about 180kgf per hull (say 1800N).

You can then use JavaProp to get an idea of the power required:
http://colaco.freeshell.org/mhepperle/javaprop/jp_applet.htm
I can help you with this if you like. It works from first principle so has wide application and gives good results. It does assume the prop is aligned with flow so there is a little variation with this factore.

From my own guesstimate of your prop size and hull dimensions, you should be able to get near your 20kts with the installed power. Would be interesting to know what it did when new.

The things that will knock performance is extra weight, dirty bum (as you have observed), extra underwater bits like that odd looking keel, engines not delivering what they should because they are need overhaul, props not selected to match the boat, huge windage on a large cabin (but you did say going downhill) etc.

Rick W.

 

When you antifoul, take a look at http://www.coppercoat.com/about.htm

Pericles

 

Thanks to everyone for the advice and to Rick for that site which was exactly what I was looking for - from that my reading is I need 75hp/side to achieve the desired 20-ish kt max.

According to the log she achieved 13kt when launched which increased to 17 after the props were cropped and cupped. I bought her soon after however and have never had the GPS rise above 14kt.

I've progressed things considerably in the last couple of days and the boat comes out today for the antifoul job. It won't be coppercoat ($$$) but it will have a 2-year warranty which is OK for me. My "financial controller" is a bit disenchanted with this boat so by then I may have been allowed to trade up and the next owner can look at a better system.

On the power front - I've gone to the prop engineers and had them calculate for me and the "ideal" results are 100hp/side driving 18" props (rather than the 15" currently fitted) turning at 1800RPM. What I need to check on now is that I have the required tip clearance for the larger props.
Using that data I've approached engine suppliers and am now waiting prices for a pair of 82hp IVECO (remember, that is a 64% increase - and the prop guy is happy with that providing I accept about a knot less which I do), 87hp Lombardini, 100hp FNM and 120hp FNM.

The weather here is PERFECT at the moment as we enjoy an El Niña summer so I plan to use the boat while we can and the next few weeks to make decisions and plan for a refit around May/June (our winter).

Once again, thanks to everyone for their interest and support.

Cheers! Alan

 

Hi Guys - Just in case anyone is interested:
Ron Given (the designer) came up and met me today where the boat is on the hardstand having just recieved a newly painted bum. His advice is to tread lightly and do things in stages.
1. Fair off the skeg (keel) at the stern to more of an aerofoil section - as Rick noted this is creating drag and turbulence
2. Bog (epoxy-fill) struts to a "fair" profile instead of the hard corners and chunks presently there
3. Flatten the hull V bottom aft of the shaft and make room for larger props

Sea trials

4. Source some 17" props and install

Sea trials thru summer whilst considering replacement engines

Although these measures probably won't get me close to my desired performance, he feels that each of the tiny improvements combined should make a noticeable difference. FINGERS CROSSED!

 

You will have a ready audience for results of the sea trials - good luck. Much beter if you can get useful gains from overall efficiency improvements rather than just going bigger and needing to burn more fuel.

Rick W.

 

You can say that again Rick. A boat like that can cost hundreds on fuel just for one good weekend of fishing.

 

A quick update following sea trials after a re-antifouling, PropSpeed to the props, rudders, struts etc. and reinstalling the shafts into the gearbox couplings.
A very pleasing:
~ WOT 14.2kt @ 3500RPM
~ Cruise 12.5kt @ 3200RPM
~ Consumption 12.5ltr/hr each engine

I'll haul her next week over our National Day to fair off the keels and struts and see what affect that has and meanwhile I'll be investigating some replacement props ahead of reshaping the bottom which is a little more scary!

More to come . . . . . .

 

2 seasons and many $$$$ later . . . .

Engines replaced with 2 x 100hp Nissan driving 17" props and a complete paint strip and refinished snow white with dark titanium metallic topsides (IMHO!).

We've just returned from a week of cruising/extended trials and she performs exactly as I originally wanted - cruise 16kt, WOT 21kt and best of all; still burning just 12.5l/h each motor.

I am VERY happy!

Cheers, Alan