Not a Design but a Use Case change: Prop changes as needed?

800 rpm = 5.29 slack tide on ICWW
1000rpm = 7.00 mph 3.2gph per motor
2000rpm =9.01mph

Does this help with analysis?

 

300 gallons fuel and 4 pax and 100 pounds of food and stuff is 2600 pounds. I'm only asking for clarity how that fits into the 15,000 figure. I said it was probably not 15,000 pounds loaded.

Another factor is engine hours. If your engines are tired; they are gonna take more fuel to produce the same result.

I apologize for commenting and not being able to provide you with a solution, but if I did a prop calc for you; it would be more unprofessional than my comment/query on hours and weight, which are only clarifying questions to help make sense of your situation.

I'm in a marina with a bunch of big cruisers and always fascinated by the horrid fuel economy they have. Most of them cruise up and down the river slow and average 1 mpg. They are all operating planing hulls at slow speeds like you.

I'm curious to see what advise you get from the real pros like baeckmo.

 

I have had a fair bit to do with big block petrol engines in boats, Chevys, Fords and Chryslers mostly. The Chevys are a big lazy lump that roar when you push them, but prefer a more sedate pace. Because they are highly loaded in boats they do not have much leeway with loading and will soon burn valves or seize pistons due to elevated combustion temperatures. I suggest no prolonged running on one engine, but changed prop type to fully submerged and set the prop load to achieve factory recommended max RPM, you will still burn a lot of fuel however. The best option really was made by fallguy, fit a four stroke outboard for slow trips, maybe 50-70hp for a boat that size. Pictures help by the way Swampy. 70 horse 4 stroke will use about 3.5 gal/hr full throttle and about 1.5 gal/hr half throttle. Very approximate and US gallons.

 

Thanks Guys

 

Will do some trial and error soon - have a set of 16x26 3 blade clevers. Boat wont plan with them but interested to see what it does to 800, 1000, 1200 rpm ranges.

 

The prop in this case is working in a strong wake after the submerged transom. In fact the water is moving forwards, ie the prop is working at a lower forward speed than the hull. This situation calls for a larger prop dia and a smaller pitch, not the other way around.

You have to realize that there are four separate factors that influence the matching; the wave system, the hull resistance characteristics, the propeller thrust characteristics and the engine brake specific fuel consumption ("bsfc").

To start with the latter, the specific fuel consumption (in US "pounds per horsepowerhour") is increasing dramatically with reduced rpm and torque (at idle it goes to infinity, since fuel is burned without any mechanical output). This means that there is a narrow operating window where the torque can be increased (by a bigger screw), leading to lower bsfc, while holding the required speed. To a large extent, this depends on the fuel mapping of the engine, and you can see that for a given (low) output, a smaller engine working closer to its max torque will always have a lower bsfc than the big engine in this operating regime. This is why practically all vessels designed for patrol work (loong loitering sessions broken by an occational sprint session) have a separate low-speed propulsion system.

But you can easily spoil the bonus by using too small a propeller, because (simplifyed) the propeller efficiency goes down, keeping the thrust load constant with smaller diameter.

I'll see if I can find some relevant fuel diagrams somewhere in my sediments; it's always easier to explain graphically.

 

Appreciate the inputs.
Because the surface drives can be lowered and are about 3 feet long, the props can run in fairly clean water behind the deep V hull at slow speeds. Your comments about load and fuel consumption are well taken. My strategy is two fold since my repower: 1) get a set of good props that are tuned to the motors WOT rpms of 5,000 and run the boat on plane in the sweet spot between 3600 and 4200 rpms and use it normally. 2) Have a set of oversized non-planing props to improve low rpm hull speed running for when I plan to go far and slow and using the autopilot. 100 mile runs to the Keys at trawler speeds when it makes sense. As mentioned, I have the oversized (pitch-wise) props to try this weekend.

 

picked up some high rake 16.5x17 3 blade rolla props. Boat now gets on plane easily, can cruise at 3800 at 24mph (one adult, 200g fuel, light water) and pull 5000 rpm with some room to spare. Hopefully now she can plane when full of fuel, water, and people and not overwork the engines.

 

....and the fuel consumption at 6 mph then?

 

3.1/motor so 6.2gph or 1 mile/gal

 

Your engines are certainly thirsty!
Would it be possible / viable to just use one engine for slow speed cruising?

 

Yes indeed. Maybe get a 25hp ob and learn to love slow.

 

I can run on one motor but only have power steering on one motor at the moment. I have pump on each but only have one hooked up....could look into changing that. Vessel will move on one motor without issue.

 

Took a few 10 mile runs on snotty seas this weekend (3-4 footers) - got 30mph nice cruise at 3900-4000 rpm at 17.5gph per motor (.857 miles/gallon). Rooster tail looked good and even at 30mph - faster, not so much. Had one arneson drive trim pump fail so was unable to "tune it" up and down as needed and if I ran motors faster the boat was listing to port pretty hard so could not test 5100rpm WOT (to confirm props are close to correct). Had 200 gals fuel, 80 gals h20, 4 large adults and two kids, plus food, gear, etc. for an overnighter. With some prop tuning and fixing trim pump I think I will improve gph with a slower rpm plane (would like to get to 3600 rpm best "eco" cruise) ... super happy the motors sounded great, not overworked (for a big boat) and temps never went over 172F. Promising results!

 

Excellent, good when a plan works out.