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jm21 · 22-05-2012, 15:56

Some folks on this board are real naysayers...I say it's a great idea and even if it doesn't work out, kudos for putting your money where your mouth is. I will be very interested in the results. I suspect it's far more than enough power for the boat and won't change the low-speed handling characteristics at all. But might be harder to handle in really big waves.

Why shoot for 8 knot cruise on a boat that size though? I would think 6-7 knots would be a much more efficient speed.

Astral Blue · 22-05-2012, 22:28

Quote:

Originally Posted by jm21

1. Some folks on this board are real naysayers...I say it's a great idea and even if it doesn't work out, kudos for putting your money where your mouth is. I will be very interested in the results. I suspect it's far more than enough power for the boat and won't change the low-speed handling characteristics at all. But might be harder to handle in really big waves.

2. Why shoot for 8 knot cruise on a boat that size though? I would think 6-7 knots would be a much more efficient speed.

1. I am humbled, but I can't take credit for putting my money where my mouth is. The data I used and the calculations derived (such as that from boatdiesel.com and David Gerr's formulas) from it have been developed by highly experienced naval architects with extensive engineering knowledge. I'm depending on their insight to give me accurate information. Fortunately, their knowledge has a track record of proving itself to be valuable.

I don't feel what I'm doing is original or covering territories that have not been explored. I'm certain this has been done numerous times and seldom discussed on these forums. I agree it is nonconventional; and anything nonconventional is going to be a magnet for naysayers. So let it be. After the naysayers have exhausted their breath, they will move to the next project they can receive false gratification from criticizing. Such is life.

2. You're right about 6-7 knots being more efficient. At 7 knots, my fuel burn rate is .8 gallons per hour; and at 8.5 knots, it is 1.2 gallons. Going from 7 knots to 8.5 knots, my fuel burn increases by 50%! The answer is clear.

I will be installing a new Floscan meter shortly...one that is intended for Diesel engines. I cannot use my old one, which was for gasoline engines -- as Diesel engines have a return line and thus need an additional sensor and a reconfigured controller to account for the difference between the feed and return.

After getting the Floscan installed and calibrated, I will have actual data to enable me to determine the most efficient cruising speed. Since my hull speed is 6.7 knots, I'm guessing my most efficient cruising speed will be right around there.

Tony B · 23-05-2012, 05:40

From the time you put your engine in gear at a slow idle up until you reach just below theoretical hull speed, your fuel economy goes downhill in almost a straight line progression. It takes a slightly bigger drop at hull speed as you build a small wave in front of you and try to climb it (more water to push). Your MPG then slowly levels out as you climb your own bow wave and start to plane.
Bottom line - the faster you go the more it costs per mile. Each boat reacts somewhat differently but the basic mechanics of it are the same.
If you use your boat enough, the relative high cost of the FloScan will pay for itself. I could see it being invaluable if for some reason you found yourself trying to beat out bad weather on a limited amount of fuel.The FloScan coupled with a GPS will allow you calculate how fast you can go on that limited amount of fuel without running dry.

Astral Blue · 23-05-2012, 08:09

Quote:

Originally Posted by Tony B

From the time you put your engine in gear at a slow idle up until you reach just below theoretical hull speed, your fuel economy goes downhill in almost a straight line progression. It takes a slightly bigger drop at hull speed as you build a small wave in front of you and try to climb it (more water to push). Your MPG then slowly levels out as you climb your own bow wave and start to plane.
Bottom line - the faster you go the more it costs per mile. Each boat reacts somewhat differently but the basic mechanics of it are the same.
If you use your boat enough, the relative high cost of the FloScan will pay for itself. I could see it being invaluable if for some reason you found yourself trying to beat out bad weather on a limited amount of fuel.The FloScan coupled with a GPS will allow you calculate how fast you can go on that limited amount of fuel without running dry.

Next to my GPS, the Floscan has been my most valuable navigation tool. It paid for itself very very quickly.

Cheechako · 23-05-2012, 08:28

Quote:

Originally Posted by Tony B

I think we get side tracked here from time to time. We keep comparing it to a full displacement hull when what we should be doing is compare it to itself. The boat has not changed. It is the same boat. So, the real question is can a 48HP diesel engine push this boat at 7 MPH? I think a 9.9HP outboard could push it to that speed.
My guess is that the boat weighs in at 6 ot 7,000 lbs. That equates to 3 1/2 tons.
Frome everything I read, it takes 2-3 HP per ton to push a vessel at almost hull speed. Lets assume the 2 is for an easy moving hull like a full displacenment so we wont use it. Lets go with the 3 HP/ton which would be closer to what we need and we get only 10.5 hp required. Even if we went to 5 HP/Ton it would still only need 17.5 HP to move it to about 7MPH. He has 48HP in there now. The boat is the same boat, nothing has changed except the motor and less weight. Under the worst of conditions he can replace it with lead, steel, concrete or 100 gal worth of water tanks or 500 cans of Campbells Soup. That's under the worst of conditions. Under the best of conditions, the stern is now much lighter and most claim that that model has too heavy of a stern to begin with. Given that, he migh be better off. In reality, he will prolly end up somewhere in the middle and still have enough HP to spare.
If you dont think a 9.9 HP OutBoard could push that boat at 7 MPH then the discussion is over and all we can do is wait for the in-the-water test results.

Yeah, I said this early on... 48 hp is way overkill to gain hull speed, flat transom or not ... it's not going to be an issue. However, control when steering with the I/O drive may not be great for sure. And the 48 HP is not going to make it plane in any way. OTOH, heck a Harley has 48 horspower and doesnt really need it either! But it sure sounds good... :>)

Cheechako · 23-05-2012, 08:34

Quote:

Originally Posted by Tony B

What makes this thread so interesting is that none of us are Naval Architects and we are just speculating.
He currently runs the boat now and has been running it at around 7 MPH so I dont see anything changing there. If it was squirrelly before the motor change it will be after the motor change. If it wasn't squirrelly before the change it shouldn't be now except for the weight change, and that could be easily corrected by adding weight.
Oh well, I will patiently wait for this project to finish.

In my experience, many Naval Architects are speculating also! It's as much an art as a science in the real world....

Tony B · 23-05-2012, 08:41

I ran into a guy on a Mainship forum that did the Great Loop. He meticulously logged over 5,000 miles with his FloScan in an identical boat that I am in the process of buying. He sent me a copy of the graph he made. It is a twin gas engine boat and it is amazing how much difference there could be in fuel consumption when running 1 to 1 1/2 kts slower than hull speed. Also a major difference in MPG running one engine at a time at that slower speed. Most people just dont want to travel at 6 - 7 MPH. I'm coming from a sailboat so I am used to it. When I get past the survey next week, a FloScan is on the top of my list of 'must have' toys for the boat.

wmcruiser · 02-06-2012, 23:20

HI, i have just carried out a similar conversion-
I have a rinker fiesta vee cruise 23ft 6000lbs that had a 5.0l v8 petrol and removed (due to corrosion of unit) and repalced with 1.8 ford diesel direct to stern drive.
the engine transplant went well, but prop resizing took some doing as getting low enough pitch to obtain RPM-
ended up with outboard prop repitched by local engining company to 14 x7P.(as not using through prop exhaust refitted with waterlock set up to transom exhaust)
this gives max rpm 3200 (9Kts)and cruisng of around 6-8 (2600rpm)kts depending on tide etc.
fuel burn between 5-9 ltres/hr depending on conditions but much less 4lt/hr @ 4-6 kts.
Good to fair weather/sea conditions(force 1-3) handles very well but may add a rudder safe unit to help with steerage.
bad sea condition (f 4-5 wave roll height 1-1.5M) which i got caught in which wasnt palnned or predicted before leaving - very interesting she powered though ok but ride was very rough/rolling as managed min 6kts speed but really need more to push though. got back safely but would recommend it for faint hearted (or intend to do it again !!!!).
next thing to go is to look at ballst issue as bow now heavy due to 200kg engine weight loss at stern. this may improve speed a little on good days allowing bow to lift more than it does.

Astral Blue · 03-06-2012, 20:58

Quote:

Originally Posted by wmcruiser

HI, i have just carried out a similar conversion-
I have a rinker fiesta vee cruise 23ft 6000lbs that had a 5.0l v8 petrol and removed (due to corrosion of unit) and repalced with 1.8 ford diesel direct to stern drive.
the engine transplant went well, but prop resizing took some doing as getting low enough pitch to obtain RPM-
ended up with outboard prop repitched by local engining company to 14 x7P.(as not using through prop exhaust refitted with waterlock set up to transom exhaust)
this gives max rpm 3200 (9Kts)and cruisng of around 6-8 (2600rpm)kts depending on tide etc.
fuel burn between 5-9 ltres/hr depending on conditions but much less 4lt/hr @ 4-6 kts.
Good to fair weather/sea conditions(force 1-3) handles very well but may add a rudder safe unit to help with steerage.
bad sea condition (f 4-5 wave roll height 1-1.5M) which i got caught in which wasnt palnned or predicted before leaving - very interesting she powered though ok but ride was very rough/rolling as managed min 6kts speed but really need more to push though. got back safely but would recommend it for faint hearted (or intend to do it again !!!!).
next thing to go is to look at ballst issue as bow now heavy due to 200kg engine weight loss at stern. this may improve speed a little on good days allowing bow to lift more than it does.

It is great to hear from someone who has performed a repower of this nature and has done so successfully!

My project is well under way but is moving rather slowly. I have performed extensive calculations and have determined a 16X10 propeller is best suited for this configuration. Fortunately, Volvo Penta makes a 16X9 propeller for my drive, which should be sufficient.

I am very curious about your waterlock. I have considered installing one but have been concerned about the potential horsepower loss. On a 48 horsepower engine, I cannot afford it. Have you tried running it without the waterlock to see whether there is a difference? At this point, I am planning on having the exhaust exit through the transom at a few inches below the waterline. That way, I will experience no or very little power loss and still have somewhat adequate sound muffling.

The fuel burn figures you noted are consistent with the calculations I made. I'm looking forward to seeing whether the calculations materialize when I begin collecting data after the project is complete.

Cheechako · 04-06-2012, 08:59

I am very curious about your waterlock. I have considered installing one but have been concerned about the potential horsepower loss.
you have a lot of HP you arent going to use anyway....

Astral Blue · 04-06-2012, 10:31

Quote:

Originally Posted by Cheechako

I am very curious about your waterlock. I have considered installing one but have been concerned about the potential horsepower loss.
you have a lot of HP you arent going to use anyway....

Not really. My effective horsepower potential is 48 at the prop shaft under wide open throttle. At a cruising RPM of 2600 RPM, I'm at about 35 horsepower. Losing 3 horsepower would constitute about a 10% power loss, which is significant. This is especially of concern if I'm going against a current. Not to mention, my alternator is going to be taking another 3 horsepower when it charges.

Cheechako · 04-06-2012, 11:10

hmmm... i would guess without spending a bunch of time that a 6 hp outboard would push your boat at hull speed. Yeah you want some extra for situations... but you're not going to plane anyway right? I've put 8hp 4 strokes on 26 foot heavy built boats as kickers and they would push the boat at Disp. hull speed in a light chop.... with an extra 30-40 Hp you should be good to go right? a 4-108 pushed my med/heavy displacement 44 footer to hull speed at far less than max rpm in strong currents throughout Wa state and Canada.....

zeehag · 04-06-2012, 11:25

my deep keel heavy displacement cruiser with a heartshaped flat transom loves to SAIL in 60+kts of breezes and pushes herself thru the seas at 8.4 kts under reefed mizzen and reefed jib. engine off. most sloops with fin/spade or fin/skeg combinations chicken out of sailing in over 30 kts breezes. you do the math. when a chubasco or other sudden wind pipes up, what would you rather be sailing?? i know what i would rather sail and surf with--and that is my formosa 41. stabiliity?? in those nice breezes we heeled 5 degrees only. match that in a sloop or any kind of light boat rig. i am not speaking of racing boats--i m speaking of comfortable cruising, getting there without the exhaustion factor and being happy while cruising.