Reply
 
Thread Tools Display Modes
 
Old 07-25-2012, 12:59 PM   #21
Guru
 
Nomad Willy's Avatar
 
City: Concrete Washington State
Country: USA
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 13,707
As to the power requirement consider carefully how much of the power you used on the old engine and how that loaded the engine. How important engine loading is, is not really clear but engine manufacturers have the best information .... especially the engine you will have. I personally prefer 55 to 65 % of WOT determined by a percentage of the maximum fuel that the engine can burn at rated rpm. Get that max fuel burn number from the engine manufacturer. A 120 Lehman is 6 gph for example. As I recall most of our engines are most efficient at about 1800 to 2000 rpm. Over propping may produce even more efficiency but has it's own problems. Many do it though. Running your engine about 600 rpm down from it's rated speed would be a good load to shoot for if you can attain max rater rpm .... And you should be able to do so. It's quite likely a smaller engine would be better for you as most boats are over powered. I think a smaller engine running at a greater load is better but many disagree w me. I also think one needs very Little (if any) extra power for bad weather ect. The most objective people to talk to about this aren't on the forum ..... Manufacturers!
__________________
Advertisement

Nomad Willy is online now   Reply With Quote
Old 07-25-2012, 07:00 PM   #22
Senior Member
 
stevensibs's Avatar
 
City: Rockport
Country: USA
Vessel Name: Still Sibsie
Vessel Model: 42' Bristol Trawler
Join Date: Mar 2011
Posts: 157
Very insightful...I agree with you that a smaller DIESEL engine running for long periods at 65 to 85 per cent of it's rated power is better than a larger engine with added horsepower that never gets used. I never got to see this boat run; the nice old folks that I bought it from lived on her for some 20 years then put her up on the hard waiting to be sold. I considered rebuilding the old Cummins but could not get parts or support down the road. I still may spring for the Lugger but it messes up my budget fierce. My feeling is anything from 160 to 210 hp will do the trick and as you say I can dial it in by bending the wheel.
__________________

__________________
Steve and Sibsie
42' Bristol Trawler
member ABYC
www.sibsie.com
stevensibs is offline   Reply With Quote
Old 07-25-2012, 07:33 PM   #23
Guru
 
koliver's Avatar
 
City: Saltspring Island
Country: BC, canada
Vessel Name: Retreat
Vessel Model: C&L 44
Join Date: Nov 2007
Posts: 2,166
That sounds like a question for the Forum on "Boatdiesel.com" Spend $25 for a membership. It will be the best investment you can make before repowering.
koliver is offline   Reply With Quote
Old 07-25-2012, 07:39 PM   #24
THD
Guru
 
City: Seattle
Country: US
Join Date: Apr 2012
Posts: 1,142
You might want to do a search for recent articles by Steve D'Antonio. He wrote recently (in Passagemaker I think) that virtually all displacement hulls are way overpowered. He is pretty compelling on the issue. You also may be able to find it on his website www.stevedmarineconsulting.com. I have talked to him on several occasions and he is a pretty helpful, accomodating guy. It may turn out you only really need a much smaller powerplant than you think.
THD is offline   Reply With Quote
Old 07-25-2012, 08:02 PM   #25
Senior Member
 
stevensibs's Avatar
 
City: Rockport
Country: USA
Vessel Name: Still Sibsie
Vessel Model: 42' Bristol Trawler
Join Date: Mar 2011
Posts: 157
I will join Boatdiesel.com and I have corresponed to Steve D. Good ideas.
__________________
Steve and Sibsie
42' Bristol Trawler
member ABYC
www.sibsie.com
stevensibs is offline   Reply With Quote
Old 07-26-2012, 01:38 AM   #26
THD
Guru
 
City: Seattle
Country: US
Join Date: Apr 2012
Posts: 1,142
Keep us posted on what happens. It will be interesting to see what you come up with ccompared to what you originally had.
THD is offline   Reply With Quote
Old 07-26-2012, 07:54 AM   #27
FF
Guru
 
FF's Avatar
 
Join Date: Oct 2007
Posts: 16,518
the nice old folks that I bought it from lived on her for some 20 years then put her up on the hard waiting to be sold.

If those nice old folks kept a log book , simply use the GPH as an estimate of the power required.

With a large engine that is probably not efficient at cruise I would use each 1 GPH to estimate 15 hp at the prop.

FF
FF is offline   Reply With Quote
Old 07-26-2012, 01:05 PM   #28
Guru
 
Nomad Willy's Avatar
 
City: Concrete Washington State
Country: USA
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 13,707
Twiisted71'
No. And I was of the opinion that hull speed was attained just as the bow started to rise for decades. It seemed obvious to me but if you calculate your hull speed and run your boat in near calm conditions it becomes clear that the bow starts to rise long before hull speed is reached. Iv'e known for a time now that the best speed to run a FD hull and probably a SD hull also is well below hull speed. I know of no specifics but .9 knots below hull speed must be about right. Prismatic coefficient, aspect ratio and many other things enter into it but if you like rules of thumb 90% of hull speed would seem to be about the best speed for economy. Could'nt go far wrong. Perhaps Skipper Dude is listening. On BoatDesign.net one person proclaimed that a boat was planing when the water cleanly leaves the stern. Getting back to the bow rising something more specific may be happening than w hull speed. The boat is clearly balanced on the bow wave and the following wave before the bow starts to rise and after it does the bow is beyond a doubt starting to climb or try to climb the bow wave. The surfing action I alluded to before must drop off rather quickly after the bow starts to rise. I think you are right that this is an important speed to recognize and try to understand. It would be nice to know how 90% of hull speed compares w the exact speed when the bow starts to rise. Anyone have any knowledge or opinions here. Perhaps someone could make some runs and come up w numbers that would apply?
Nomad Willy is online now   Reply With Quote
Old 07-26-2012, 07:23 PM   #29
Scraping Paint
 
City: -
Country: -
Join Date: Oct 2007
Posts: 13,748
Eric--- I don't know squat about hull design and coefficients and buttock lines and so forth. But I can give you an observation I deliberately made one day some years ago when we left Bellingham with the bay in absolute glassy conditions.

I was curious for no particular reason that I can recall so I taped a pencil horzontally to the steering cable raceway that goes up to the flying bridge infront of the helm so I could line up the pencil point with the top fo the bow rail. Keeping those things in alignment I then began to add power from our idle speed. And you know what? The bow started to come up immediately as observed againt Lummi Island across the bay. Not a lot and as I added power on up to our cruising rpm of about 1600 the bow did not rise significantly, but it rose a bit more with every addition of power.

I attributed this to the simple increase of hydrodynamic pressure against the hull. It's going faster, the pressure is greater against the hull, particularly its forward section, and the force has to result in something, so the bow comes up.

At 1600 and at the pitch the props were then (their pitch is different today) we got 8 knots. Which I believe is a wee bit over hull speed for this boat.

I've observed the same thing in the floatplane. As you idle around at what we call "displacement" speed, as soon as you start adding power, even just a bit, the nose starts coming up. Oviously when you go to runup power and later takeoff power the nose comes way up. But the nose starts rising even with an additional 50 rpm over idle. Again, I attribute this to the simple increase in hydrodynamic pressure against the floats.
Marin is offline   Reply With Quote
Old 07-26-2012, 07:49 PM   #30
Guru
 
Nomad Willy's Avatar
 
City: Concrete Washington State
Country: USA
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 13,707
Marin,
Never heard of a boat that rose in the bow just above idle. Perhaps you've just got too much power.
As to the airplane that's obvious .. You increase throttle any amount and you increase prop wash and that goes over the horizontal stabilizer and the negative incedense therein pushes down on the tail and since it is attached to the fuselage the "bow" starts rising immediately.
Nomad Willy is online now   Reply With Quote
Old 07-26-2012, 08:39 PM   #31
Scraping Paint
 
City: -
Country: -
Join Date: Oct 2007
Posts: 13,748
No, the nose of the floatplane wil rise even if you hold down elevator. Particularly right at idle when the propwash is minimal. Eventually when propwash becomes very strong the upward force of air against the down elevator will begin to counter the hydrodynamic pressure that's building against the bottoms of the floats although even with full down elevator the plane will still come up on the hump with a lot of power. It gets a little ugly after that, though, if you continue to apply down elevator. It's all about the hydrodynamic pressure on the floats. I learned this from my good friend Jay Frey who for decades was president of the float division at EDO.

Our boat has no more power than any other twin GB36 and less than many, and at the time the props had the same diameter and pitch as other GB36s with similar power and 4-bladed props. So I suspect that if one did the same "test" I did with another GB36 they'd have the same results.

I think it's inevitable, frankly. You move something faster through the water and you're going to generate more hydrodynamic pressure against the hull. And Newton says for every action there is a re-action.

The Gikumi that I was on a few weeks ago does exactly the same thing. This is the boat I've posted some shots of recently. We were idling along, a big herd of Pacific Whitesided dolphins came along, and the skipper started opening the throttle of the boat. I was standing on the bow and I felt the bow start to lift the moment the engine speed began to increase. And the Gikumi is the very definition of a displacement boat.
Marin is offline   Reply With Quote
Old 07-27-2012, 02:27 AM   #32
THD
Guru
 
City: Seattle
Country: US
Join Date: Apr 2012
Posts: 1,142
I am not sure what happens a idle or just above idle speeds, but as you approach hull speed, the bow rise is actually a stern fall, the stern "squatting" as the boat attempts to overcome the increasing drag created by larger bow waves. In a planing hull, this is when the boat "gets over the hump" i.e. transitions from in the water to on top of the water. If you have ever been in a planing boat that has toruble getting on plane, the bow goeas way, way up as the bow wave increases but the drag creates a "hole" in the water at the stern.
THD is offline   Reply With Quote
Old 07-27-2012, 10:02 AM   #33
Guru
 
psneeld's Avatar
 
City: Avalon, NJ
Country: USA
Vessel Name: Freedom
Vessel Model: Albin 40
Join Date: Oct 2011
Posts: 15,877
Quote:
Originally Posted by Marin View Post
No, the nose of the floatplane wil rise even if you hold down elevator. Particularly right at idle when the propwash is minimal. Eventually when propwash becomes very strong the upward force of air against the down elevator will begin to counter the hydrodynamic pressure that's building against the bottoms of the floats although even with full down elevator the plane will still come up on the hump with a lot of power. It gets a little ugly after that, though, if you continue to apply down elevator. It's all about the hydrodynamic pressure on the floats. I learned this from my good friend Jay Frey who for decades was president of the float division at EDO.

Our boat has no more power than any other twin GB36 and less than many, and at the time the props had the same diameter and pitch as other GB36s with similar power and 4-bladed props. So I suspect that if one did the same "test" I did with another GB36 they'd have the same results.

I think it's inevitable, frankly. You move something faster through the water and you're going to generate more hydrodynamic pressure against the hull. And Newton says for every action there is a re-action.

The Gikumi that I was on a few weeks ago does exactly the same thing. This is the boat I've posted some shots of recently. We were idling along, a big herd of Pacific Whitesided dolphins came along, and the skipper started opening the throttle of the boat. I was standing on the bow and I felt the bow start to lift the moment the engine speed began to increase. And the Gikumi is the very definition of a displacement boat.
I agree...most boats except the extreme hull designs which I have less experience with will rise as you add power.
psneeld is online now   Reply With Quote
Old 07-27-2012, 01:13 PM   #34
Guru
 
Nomad Willy's Avatar
 
City: Concrete Washington State
Country: USA
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 13,707
But the bow won't rise until one gets to within about a knot of hull speed. The gentleman that thought hull speed was at that point certainly wasn't referring to a speed or throttle setting "just above idle".
PS ... When I say "bow rise" I'm referring to the fore and aft angle of the whole boat ... Not just the bow. Clearly the bow rises and the stern drops.
I'm still hoping someone will make a run and observe the speed.
And Marin I was talkIng about the horizontal stab ... Not the elevator.
Nomad Willy is online now   Reply With Quote
Old 07-27-2012, 01:31 PM   #35
Scraping Paint
 
City: -
Country: -
Join Date: Oct 2007
Posts: 13,748
Last time I looked the elevator is attached to the back of the horizontal stabilizer and its power will way overcome any effect air moving over the stabilizer portion of the empennage might have. So if the air moving over the stabilizer puts a downward force on it, putting the elevators down will negate the stabilizer's downward force by a major big factor.
Marin is offline   Reply With Quote
Old 07-27-2012, 01:46 PM   #36
Scraping Paint
 
City: -
Country: -
Join Date: Oct 2007
Posts: 13,748
Quote:
Originally Posted by manyboats View Post
But the bow won't rise until one gets to within about a knot of hull speed. When I say "bow rise" I'm referring to the fore and aft angle of the whole boat ... Not just the bow.
I think it would be quite a trick to raise the bow but not change the fore and aft angle of the boat. The bow and the rest of the boat are sort of attached to each other.

The point you seemed to be making earlier is that the bow will not rise until hull speed starts to be exceeded. I agree that the attitude of the hull will change at that point as the hull begins to dig a deeper hole in the water and the stern sinks into it. But if your point is that the hull will ride perfectly level, or at static trim, unitil that speed is reached, from my observations it doesn't.

As soon as power is added and the hull begins to move forward, its forward motion against the water begins to generate hydrodynamic pressure against the hull. It's physics--- it has to. And that pressure is going to move something. Some of that pressure is disspipated back into the water itself. But some of it is going to lift the hull. Not a lot, but some. Hence what I observed in my crude pencil test and what I felt on the bow of the Gigkumi. In both cases, the bow began to rise the moment the boat began to accelerate even though that rate of acceleration was very slow.

More speed, more pressure, bow is forced up.

How all this relates to the wave produced by the hull and why it "squats" as the speed increases I have no idea. All I'm saying is that as soon as a hull begins to move through the water there is pressure against the parts of the hull that are meeting the water and this pressure begins to lift the hull where the pressure is greatest, which on a conventional design is the front end. So the bow comes up. Our GB does it, seaplanes do it, our 17' Arima does it, etc.
Marin is offline   Reply With Quote
Old 07-27-2012, 05:28 PM   #37
Guru
 
Nomad Willy's Avatar
 
City: Concrete Washington State
Country: USA
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 13,707
As to the power requirement consider carefully how much of the power you used on the old engine and how that loaded the engine. How important engine loading is, is not really clear but engine manufacturers have the best information .... especially the engine you will have. I personally prefer 55 to 65 % of WOT determined by a percentage of the maximum fuel that the engine can burn at rated rpm. Get that max fuel burn number from the engine manufacturer. A 120 Lehman is 6 gph for example. As I recall most of our engines are most efficient at about 1800 to 2000 rpm. Over propping may produce even more efficiency but has it's own problems. Many do it though. Running your engine about 600 rpm down from it's rated speed would be a good load to shoot for if you can attain max rater rpm .... And you should be able to do so. It's quite likely a smaller engine would be better for you as most boats are over powered. I think a smaller engine running at a greater load is better but many disagree w me. I also think one needs very Little (if any) extra power for bad weather ect. The most objective people to talk to about this aren't on the forum ..... Manufacturers!
Nomad Willy is online now   Reply With Quote
Old 07-27-2012, 06:07 PM   #38
Guru
 
Nomad Willy's Avatar
 
City: Concrete Washington State
Country: USA
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 13,707
Never observed what the bow does attitude wise just above idle. No reason to either. But the bow does reach a point 1/2 to 1 knot below hull speed where it distinctly rises. That's the event I'm talking about. Quite a number of people think the bow rising event is the magic hull speed event ... Not so but it could be even a more important speed to be aware of.
But different hulls will probably have the event at different percentages of hull speed. A banana shaped hull having less pitch stability than a boat more typical of most trawlers w relatively plumb stem and full bows w large, flat and straight aft sections. They resemble cars w long wheelbases. So ther'e may be too many variables to apply specific numbers to specific events but the specific bow rising speed for a specific boat could be close enough to be an excellent rule of thumb. I'm hauling my boat in a few days so I'll not be able to make any specific observations but any number of boat owners here could play this out. In a week or so I'll be going back to Alaska for a month or so.
Nomad Willy is online now   Reply With Quote
Old 08-01-2012, 06:18 PM   #39
JAT
Guru
 
JAT's Avatar
 
Country: US
Vessel Name: Just a Tinch
Vessel Model: Gulfstar 44 MC
Join Date: Jan 2010
Posts: 595
I know this really doesn't fit here...but I bought a 2004 Dodge Ram 3500 Dually with the 5.9L Cummins Diesel....thought I was being patriotic...turned out the truck was "Hecho en Mexico".... It was a new Cummins engine...but in the 8 years that we owned the pickup, I never had a single problem with it. Engine, tranny or body....sold the truck for almost half what I paid for it..... Being reman'd or recon'd in Mex under the guidance of Cummins.... I would have no problem buying that engine..... Would prefer one done in the US simply because I am a tad xenophobic....
JAT is offline   Reply With Quote
Old 08-02-2012, 11:04 AM   #40
TF Site Team
 
ksanders's Avatar
 
City: SEWARD ALASKA
Country: USA
Vessel Name: LISAS WAY
Vessel Model: BAYLINER 4788
Join Date: Feb 2011
Posts: 3,952
As a FYI to the OP.

There was a great article in Passagemaker magazine a year or so ago about the Cummins reman facility in Reynosa Mexico. You might contact passagemaker and get a copy of that article.

When we repowered our boat we opted for the Cummins Reman engines, in part based on that article. Their facility is pretty impressive.
__________________

ksanders is offline   Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off





All times are GMT -5. The time now is 08:42 PM.


Powered by vBulletin® Version 3.8.8 Beta 4
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2006 - 2012