Improving Efficiency at 15-20kt

I have a 25' Pacemaker SF, with a round-bilge, semi-displacement hull. The vessel is powered by a 5.7L V8 inboard, driving a 13LH10 cupped, 3-bladed propeller via a 1:1 transmission. While I am generally pleased with the boat and I think it is a great all-around vessel for a large variety of purposes, I am dissatisfied with the efficiency. At present, I typically cruise at 15-19kt (depending upon load and conditions) at engine speeds of 3000 - 3500rpm. My economy at these cruise speeds ranges from 1.1kt/gal. to 1.5kt/gal., again dependent upon load and conditions.

My goal is to try and markedly improve the efficiency of the vessel at a cruise speed of 15-20kt. Top speed is unimportant. Improved efficiency at hull speed would be icing on the cake. I have been considering a few potential ideas, and I would love to hear some comments...

1) Could I expect to see a dramatic improvement in efficiency with the addition of a reduction transmission, coupled with a larger propeller with greater pitch? As I understand it, a larger diameter propeller is typically more efficient. In addition, I belive that a square propeller (e.g. one with roughly proportional diameter and pitch) is mathematically capable of acheiving greater efficiency than one with less pitch. Assume for the sake of argument that the vessel can be made to accomodate the larger propeller (up to 16" diameter or so).

2) Would the addition of chine rails help improve the performance on plane, without significantly degrading displacement speed efficiency? I have heard that one of the primary failings of the round-bilge semi-displacement hull is greater water resistance, as the water doesn't break free from the hull cleanly at the chine (as is the case with a hard chine boat).

3) Pacemaker replaced this round-bilge hull with a warped plane, hard-chine semi-displacement hull in the mid-seventies. All things being equal, how much better would one expect a hard chine, semi-displacement hull to perform compared to a similarly sized round bilge, semi-displacement hull? How much worse would this hull perform at displacement speeds?

4) Of all the used boats one might be aware of in this size and general layout, what examples could one give of optimally efficient hulls at cruise speeds in this 15-20kt range? Would a planing hull with constant deadrise (a medium vee) be a better choice? Who produced such a hull?

PS: I think the keel of this Pacemaker hull is a nice thing to have in Florida...grounding is always a concern.

I look forward to any comments anyone cares to offer.

Regards,

joebobb

 

I've installed spray/lift rails in boats of this type. They improve top speed but create drag at lower speed. The economy of your boat is about as expected. Reducing weight by taking out unnecessary stuff is usually the best and cheapest way of increasing economy.

 

Gonzo is bang on the money. Weight reduction is by far the most effective and cheapest way of improving efficiency. There are other alternatives of course - you could replace your engine with a diesel which would probably reduce your consumption a little - but I'd bet that you'd never put enough hours on the boat to justify the expense.... a few extra bucks each fill is a hell of a lot cheaper than 30K for a new engine.
Trim tabs might give a marginal improvement - then again the increased drag may more than offset any improvements brought about by a lower running trim.

It's true that a hard chine hull is more efficient at planing speeds (say 20+ knots), but take a look at the original Midnight Lace series design by Tom Fexas. They all featured round chine hull-forms and were very efficient at the 15-20 knot range that you're after. Quite apart from the fact that they were relatively narrow by todays standards, they all featured one thing in common - they were light....and bingo, we're back to what Gonzo said...
Pull those spare beers out of the bilge, don't fill the water and fuel tanks to the brim if you're only out for the day and tell the wife that for once you're happy for her to do some spring cleaning - you'll halve your fuel consumption overnight!!

 

This requires a simple reanalysis of the propeller efficiency to determine if the prop is optimized for the speed range desired, but you have to have some trial data, propeller software and training in using it. I used just this type of problem as an example in a Professional BoatBuilder article a few years ago, and as a homework problem in my prop course course, so it's a pretty quick exercise. A naval architect will have all this at hand. Dave Pritchard is a qualified NA in Florida and can probably help you with this.

 

Thanks for the feedback thus far...

Well, I appreciate the comments received so far. Regarding weight: Yeah, I tell you I try to do the best I can at keeping extraneous stuff out of the boat, and I typically do carry only enough fuel for the journey...outbound + inbound + a safety factor, dependent upon nature of trip and destination. It can be hard to do...I don't know where we find all the crap, but take 2-3 guys out on a fishing trip, and you'll see more bags and gear than a small army show up at the dock.

One thing I did notice this weekend: I took my father and sister out boating. Neither one is a light person. They were sitting in the stern at first. Eventually they moved up to the bridge. I figured the shift in mass, forward of the CG would probably be detrimental to the performance, but the boat actually picked up about 1kt and flattened out noticeably. Moral of the story is that trim is a significant factor, and apparently the flatter the better (to a point, I guess).

For the heck of it, I tried to see what my prop slip was, by using a few of the free prop calculators available on the net. I have no idea if these calculations are of any significance; however, I was getting slip figures in the mid-40% range at 3500rpm...dropping to the mid-30% range at 4000rpm. I realize that slip does not equate to efficiency. Nevertheless, doesn't this seem a bit high? I thought slip should be around 10%, maybe less, at higher speeds. Here are the sites I looked at...

http://www.rbbi.com/folders/prop/propcalc.htm

http://www.boatramp.com/prop_applet/PropAnalyzerAppletG.html

http://www.continuouswave.com/cgi-bin/propcalc.pl

I will consider speaking to a Naval Architect on this subject. However, I have no idea how dear this type of service would be.

 

From what you've said suggests that the addition of trim tabs may improve things a little. We have an Offshore 48 Yachtfisher - a hard-chine semi displacement boat. Even as low as 10 knots, the addition of some tab picks the speed up marginally. Admittedly, we haven't run consumption figures, but the gain appears to be there nonetheless.
As far as your prop goes, a read of Dave Gerr's Propellor Handbook might give you some further insight into how things should be

 

The firm I worked for in Seattle performed an economic and fuel efficiency analysis of a factory trawler to select a nozzle and determine return on investment and billed for eight hours. This also required a lot of engine stuff.

The homework problem was supposed to take less than half an hour. Maybe you could go to FAU or FIT and get some student help.

 

Also, you can use Nordstrom-Mercier or the NPL series to get good data on the resisitance of round bilge hulls of this type and speed.

 

Hmm.....

Ahh yes, the good ol' Nordstrom-Mercier. When I was a kid, we used to rely pretty heavily on that. That is, until that damn NPL series came along. Darn whippersnappers...

Might be interesting to see if any students would be interested in fiddling with this...but I have my doubts. Nothing too sexy about improving the efficiency of a 37-year old, antiquated, hull.

 

1/1 is not enough reduction...

Semi displacement boats work better with reductions ranging from 1.8 to 3. The problem is the cost as you have to change the gearbox, the shaft, the propeller and many thigs to locate properly the bigger propeller... Unhappily there are not cheap ways to improve the efficiency of your boat. Alas, big gas V8 are not known for their efficiency.

I had been asked for a similar problem with an old Chris Craft 31 with 2 big 7.4L gas suckers and only 18 knots. The owner who used commercially this boat for sport day fishing trips had his wallet dried by the note gas, and the mechanical problems (plus a beginning of fire).

A drastic weight reduction programm was done; all the useless interior joinery has been removed (big kitchen,air conditionning, bathroom, waste tanks, etc), the heavy 2 engines, transmissions and the enormous gas tanks were thrown out. A diesel Yanmar 250 HP, with 2.4 gear box, new shaft, new propeller, small tanks, and new rudder were installed (no problem of room the engine compartment looks empty now)

Morality the diesel carburant bills went to 25% of the former gas bills with a better speed, and no more mechanician bills. But it was a pretty expensive affair.

 

for a commercial operator - or even a recreational boater who does a lot of hours each year - this can make sense. For the average boater however, this kind of extensive refit would be a case of misplaced value. Sure - you might save a thousand bucks each year in fuel, but if it cost 50K in the 1st place then you're way behind the eight-ball.

 

you wrote in a few words...

you wrote in a few words the conclusion of my post. There are not effective cheap means of improving the efficiency of a boat...

 

Cheap ways to improve efficiency...

Oh yes there are,...buy a USED Diesel engine for a start!
A used, running Diesel engine with low hours can often be picked up for the fraction of the cost of a new one, and it has an added bonus that it wont need to be run in.

Regards

Aquatek

 

What a bunch of cheapskates!
JoeBobb didn't ask for cheap ways, he asked for ways. Besides, costliness is directly related to skills and ingenuity. So I would suggest, look into a Kort Nozzle. Excerpts:
"A Kort Nozzle features a hydrodynamically shaped duct, enclosing a high thrust propeller, to minimize turbulence losses and produce exceptional thrust and efficiency. The efficiency of a Kort Nozzle can be 50% to 70% higher than an open propeller......., producing boat speeds 15% to 20% greater without using additional power.....! At a given boat speed, run times are extended 50% to 70%. The propeller is also protected against hitting stumps and rocks, is safe around swimmers and won't tangle fishing lines."
From: http://www.qis.net/~jmgraham/rr500.htm

Maybe you can learn about this and figure out how to apply it to your drive. I understand tug boats and other industrial scale boats use Kort Nozzles.

Maybe this is what CDBarry was referring to in his post.

 

I afraid the claims he gave for a Kort nozzle increasing the efficiency of an open propeller by 50%-70% are wildly exagerated!
An open propeller is no more than 69.9% efficient, adding a Kort nozzle increases the efficiency by about 17%, not 50%-70%!!!
So while a Kort nozzle will help, dont expect one to perform miracles when used with an old inefficient hull design.

Regards

Aquatek