Stability of Slow Cruising a Deep Vee Hull in Rough Water

[Cheechako]

When I worked building Alum V hull boats we tested every boat fo fuel usage. Most of the boats had outboards, either one 250 HP or two 250 HP. The efficient speed was roughly 4000 rpm at 30-40 mph. Yeah, they are sucking gas there, but you are going a long ways fast! One 250 HP was about 12.5-13 GPH, two 250 HP was about 18 GPH on the same boat. Dont let anyone tell you "you only use the Horsepower you need" when it comes to fuel efficiency...

[capngeo]

Originally Posted by Astral Blue

What were the sea conditions like? And how did the boat handle? The hull on most Bertrams in this size range is very similar to mine.

Running most engines at idle is far from their peak efficiency. If you look at your engine's performance curve, you'll see running them at 1000 RPM will not provide much torque, thus causing your engine to struggle with the load when run for extended periods under such conditions. Running one engine at a higher RPM would have most likely given you better fuel economy.

The 28 is a true deep Vee with 28° in the stern. The hull design is notoriously inefficient but rides OH SO SWEET out in the slop. Sea conditions were <2'.

I tried all sorts of combinations and the only thing I ever did to that boat to actually improve fuel economy with any significance was convert it to the Yanmars from Mercruisers. (Other than my current keeping it on the hard behind my shop... havent used a drop of fuel in 5 years!)

[Tony B]

Originally Posted by Cheechako

.....The efficient speed was roughly 4000 rpm at 30-40 mph. Yeah, they are sucking gas there, but you are going a long ways fast! One 250 HP was about 12.5-13 GPH, two 250 HP was about 18 GPH on the same boat. Dont let anyone tell you "you only use the Horsepower you need" when it comes to fuel efficiency...

I guess this is where I get confused. Running the two at once you are burning 5 to 5 1/2 gals more per hour?
At 40MPH at 13 gal/Hr = 3 MPG
At 40MPH at 18 gal/Hr = 2.2 MPG.
I'm thinking there is a difference of about $20/Hr in real money.
Where am I going wrong?

[David M]

Seakeeping is being confused with stability.

In a nutshell, stability is the vessels ability to right itself. This relates to where the boats center of gravity is in relation to its center of buoyancy, the resulting metacentric height and the shape of the righting curve that relates to the righting moment vs the angle if inclination.

Seakeeping is how a boat reacts to wind and wave conditions. Both are related yet different in a number of ways.

Planing hulls are designed to plane. This means they are not as efficient at non-planing speeds as are hulls that were designed to never plane. The wide flat after area with little deadrise that does not taper back to a point makes planing boats much less efficient off of a plane. I would think twice about downsizing your engine. You would be better off fuel consumption wise to purchase a non-planing hull if you never need to go into the double digits speed wise.

http://en.wikipedia.org/wiki/Seakeeping

http://en.wikipedia.org/wiki/Ship_stability

[Dulcesuenos]

I had this exact boat for 2 years. With the 260 and Volvo od, I personally felt it is a strictly coastal cruising design. Has tons or windage and is hard to manage in rough seas and high (35) plus kt winds. I was caught in a nasty squall for two hours and to keep her into the wind required 3/4 throttle and we made little headway.

[David M]

One more thing being confused here are two different types of efficiency being lumped into one type of efficiency.

An engines peak efficiency is never the same engine RPM where the boat is getting its greatest efficiency on a miles per gallon basis.

The slower a boat goes, the less fuel it burns per mile. This means that a boat running at near its lowst RPM is getting its best fuel economy on a per mile basis. This is nowhere near an engines most efficient use of fuel on a horsepower per unit of fuel basis, or what is called its specific fuel consumption.

To double the speed of a boat, you have to roughly cube its fuel consumption. This is why the best specific fuel consumption will never be the same engine RPM as that of a boats best fuel consumption.

The best specific fuel consumption is in a higher RPM range. The best fuel efficiency in miles per gallon because of the cubing of the consumption in order to double the speed relationship, is in a low engine RPM range.

[Cheechako]

Originally Posted by Tony B

I guess this is where I get confused. Running the two at once you are burning 5 to 5 1/2 gals more per hour?
At 40MPH at 13 gal/Hr = 3 MPG
At 40MPH at 18 gal/Hr = 2.2 MPG.
I'm thinking there is a difference of about $20/Hr in real money.
Where am I going wrong?

That's what tests on dozens of boats showed. Evinrude, Yamaha, Suzuki, and a few Hondas and Merc's. One difference is that with 500 hp you might be doing 40mph and with 250hp maybe 35 or so.. forget the actuals now......the 500hp version/26ft would do 55 mph flat out.... and a hard over turn at that speed. (seatbelt required!)

"One more thing being confused here is efficiency. An engines peak efficiency is never the same engine RPM where the boat is getting its greatest efficiency. The slower a boat goes, the less fuel it burns per mile. This means that a boat running at near its lowst RPM is getting its best fuel economy on a per mile basis. This is nowhere near an engines most efficient use of of fuel on a horsepower per unit of fuel basis, or what is called its specific fuel consumption.

Not sure this is accurate for a planing hull? they are inefficient until well up on plane. 12GPH at 36 MPH = 3.0 MPG, 4GPH at 8MPH = 2 mpg. ?

[Astral Blue]

Originally Posted by David M

Seakeeping is being confused with stability.

In a nutshell, stability is the vessels ability to right itself. This relates to where the boats center of gravity is in relation to its center of buoyancy, the resulting metacentric height and the shape of the righting curve that relates to the righting moment vs the angle if inclination.

Seakeeping is how a boat reacts to wind and wave conditions. Both are related yet different in a number of ways.

Planing hulls are designed to plane. This means they are not as efficient at non-planing speeds as are hulls that were designed to never plane. The wide flat after area with little deadrise that does not taper back to a point makes planing boats much less efficient off of a plane. I would think twice about downsizing your engine. You would be better off fuel consumption wise to purchase a non-planing hull if you never need to go into the double digits speed wise.

Seakeeping - Wikipedia, the free encyclopedia

Ship stability - Wikipedia, the free encyclopedia

David, thanks for the insight. With respect to the stability versus seakeeping terminology, I find myself conflating the two on a regular basis. You're correct. They are different but closely related.

With respect to the hull design, my hull does not have a wide flat area with little deadrise, as you described. That's what puzzles me with respect to what extent it will behave like a displacement hull. Most displacement hulls (with squared chines, that is) I've see have a greater bowrise, but resemble its pitch and angle until the stern. Where they differ at the stern is the lower and relatively flat tapering, whereas my hull ends in a steeper vee angle.

It is a slow (inefficient) planing hull but provides a very smooth ride; and I attribute it to the lack of flat surface. I could be wrong.

Originally Posted by Dulcesuenos

I had this exact boat for 2 years. With the 260 and Volvo od, I personally felt it is a strictly coastal cruising design. Has tons or windage and is hard to manage in rough seas and high (35) plus kt winds. I was caught in a nasty squall for two hours and to keep her into the wind required 3/4 throttle and we made little headway.

Yes, it does have a considerable amount of windage; and its center of gravity needs considerable improvement.

One thought I have been entertaining is to add a water ballast to increase its seakeeping ability. Any thoughts?

[David_Old_Jersey]

Am probably being a bit thick here (it happens!) - but if you were mostly cruising at 7-8 knots, then if the new engine has the power to acheive that then would not the boat handle pretty much the same as it did before at the same speed?

My gut says that the boat will be inefficient as a displacement boat and that therefore the 4108 may struggle to acheive 7 - 8 knots.

Will she be stable in rough water? - not as stable as when travelling at 30 knots (kinda like the riding a bicycle thing) and will probably make a fair bit of leeway as little or no keel, plus the windage....will she be dangerously unstable? I dunno, but I would start cautiously! but I doubt she would be ideal for heavy weather, so would plan my trips accordingly - not to say don't do the conversion, really depends on how the boat is to be used.

[Astral Blue]

Originally Posted by David_Old_Jersey

1. Am probably being a bit thick here (it happens!) - but if you were mostly cruising at 7-8 knots, then if the new engine has the power to acheive that then would not the boat handle pretty much the same as it did before at the same speed?

2. My gut says that the boat will be inefficient as a displacement boat and that therefore the 4108 may struggle to acheive 7 - 8 knots.

Will she be stable in rough water? - not as stable as when travelling at 30 knots (kinda like the riding a bicycle thing) and will probably make a fair bit of leeway as little or no keel, plus the windage....will she be dangerously unstable? I dunno, but I would start cautiously! but I doubt she would be ideal for heavy weather, so would plan my trips accordingly - not to say don't do the conversion, really depends on how the boat is to be used.

1. I began the repower project with that premise in mind. We usually cruise the boat at 7 knots; and at that speed, it is very stable. In 2-3 foot swells and moderate winds (10-12 kts), it maintains its stability quite well.

The drive had a 1.61:1 gear ratio and a 15X17 prop, which made inefficient use of my old engine at the 7-8 knot range. The drive/prop combination was better suited for pushing the boat while on plane.

Now, I have a 2:15:1 gear ratio and a 16X9 prop. With this combination, efficient use of my new engine's horsepower and torque at 2500 RPM will be made (according to calculations). Let's see how it pans out when the boat is put in the water.

2. My thoughts were the same. It wasn't until I started to make calculations on the efficiency of my hull at low speeds that I became surprised. Using the power required calculator from boatdiesel.com, I ran calculations for a displacement hull, semi-displacement hull, and a slow-planing hull at hull speed. This is assuming the weight and waterline length for my boat. The displacement hull was the most efficient, but not by much more than my hull. At 8 knots (which is above hull speed), I was astonished. It was (obviously) no surprise that a displacement hull can't achieve this in any practical manner whatsoever, but a semi-displacement hull was actually less efficient than my hull! I've attached the calculation.

Also, here is a short film of a sea trial for the engine I originally had on the boat. It was a GM 350 (Volvo-Penta AQ 260). Notice the hull is not out of the water at maximum speed. This is probably due to the absence of the flat surface David referred to earlier.

Bayliner Victoria 2750 Command Bridge on a Sea Trial - YouTube

[goboatingnow]

OP, beleive it or not a long long time ago I once had a share in a boat like that .

Firslty it's not in any way a heavy weather boat and secondly you are about to ruin it forever putting in a small diesel. It will be virtually unsellable. The boats needs speed to handle properly. Deep V hulls are very good heavy weather boats, but this isn't a deep v but it absolutely needs power in rougher conditions. All these hulls use dynamic stability of speed to aid the boat

Secondly it's an out drive , so the gearing will be completely wrong for the perkins. The engine will never be able to drive the boat at 7-8 knots without a completely custom gearbox. The only 4108 out drives I've seen were Enfield sonicdrives which are low speed out drive ( and notverygood either).

In my case the previous owner had repowered with a styr diesel and omc out drive combination which was designed as a drop in replacement. Max speed dropped to 17 knots. But was ok. The heavier diesel made her a little stern heavy

These boats were never designed to do displacement speeds and they are a PITA to stop wandering all over the place.

What you need is one of the newer high speed turbo diesels and a matching out drive.

Stop and think. Did you receive professional advice have you compared relative weights etc. a 4108 is a fine engine , noisy mind you, but it's not an example of good power to weight modern diesels.

Whoever convinced you to do this needs their head examined

If you want a displacement speed motorboat then buy one or better still buy a sailboat.

Dave

[Tony B]

Originally Posted by Astral Blue

..... I have a Bayliner ...with a 48 horsepower Perkins 4.108. .......My new engine will be able to give me a 7-8 knot cruising speed comfortably.

The boat is 27.5 feet in length .......I'm wondering whether I would be sacrificing stability in rough waters if I cruise at 7 knots. ..........
........... with a similar hull ... he would ...achieve most stability in rough seas when the boat is on plane......

I am no expert but here is my take on this anyway.
Lets start off with fuel economy. Your boat is 27.5 ft long. Without knowing your length of waterline, I will guess it at about 23 ft.
Theoretical hull speed = 1.34 X the sq. root of your waterline length.
The sq. root of 23 Ft = 4.8
So, theoretical hull speed = 1.34 X 4.8 = 6.4 MPH
Your most fuel efficient speed other than just Put-Put will be slightly under 6.4 kts....So, your most economical speed will be around 6.2 Kts. Anything faster than that will greatly increase your fuel consumption with very little increse in speed.
As for the most stability in rough seas when the boat is on plane is true - within reason. It can get too rough to plane at times. Thats a matter of trial and error. Keep in mind that the truism of this is because the boat was designed for a higher speed.
I have the Westerbeke 4107 in my sailboat - approx 22,000 lbs. and I can go 6.5 to 7.0 kts on 3/4 gal per hour. That's over 8 MPG. I suspect that you will do at least that well. What you lack in hull shape you make up for in lighter weight.
Overall, if your main objective is fuel consumption and you like cruising at a slow speed, as I do, I think you made a wise decision.

[goboatingnow]

Also, here is a short film of a sea trial for the engine I originally had on the boat. It was a GM 350 (Volvo-Penta AQ 260). Notice the hull is not out of the water at maximum speed. This is probably due to the absence of the flat surface David referred to earlier.

This is completely appropriate for a reasonably heavy cruiser on the plane. The hull is never "out of the water" its not a little bitty speedboat.

[Astral Blue]

Originally Posted by goboatingnow

1. Secondly it's an out drive , so the gearing will be completely wrong for the perkins. The engine will never be able to drive the boat at 7-8 knots without a completely custom gearbox. The only 4108 out drives I've seen were Enfield sonicdrives which are low speed out drive ( and notverygood either).

In my case the previous owner had repowered with a styr diesel and omc out drive combination which was designed as a drop in replacement. Max speed dropped to 17 knots. But was ok. The heavier diesel made her a little stern heavy

2. These boats were never designed to do displacement speeds and they are a PITA to stop wandering all over the place.

3. What you need is one of the newer high speed turbo diesels and a matching out drive.

4. Stop and think. Did you receive professional advice have you compared relative weights etc. a 4108 is a fine engine , noisy mind you, but it's not an example of good power to weight modern diesels.

5. Whoever convinced you to do this needs their head examined

6. If you want a displacement speed motorboat then buy one or better still buy a sailboat.

1. See post #25 in this thread.

2. Bow wonder begins to subside at 5 knots and is non-existent at 6 knots. There is also a device called the wonder fin, which has a solid proven track record for keeping bow wonder under control.

3. A high performance Diesel is exactly what I don't need. The Volvo D3 or D4 series engines or their equivalent runs $30K and offer very little advantage over their gasoline counterparts. I picked the Perkins 4.108 intentionally because it is a low performance Diesel engine with an excellent track record. A well maintained Perkins 4.108 can run for 8000+ hours prior to needing a rebuild. I challenge any high performance Diesel manufacturer to make that claim about their engines.

4. Yes, I did. I have the support of a large number of marine mechanics that not only understand the advantages and drawbacks of such a repower, they are aware I understand the limits of this engine. This repower is discussed extensively in the Bayliner Owners Club forums. Months of research and data gathering went into this prior to my purchasing the engine.

5. If you want to be taken seriously, please cut the insults. They undermine both yourself and your position.

6. A sailboat plays a different role than a powerboat. In this respect, they cannot be compared to one another. Sailing is definitely in our future, but we haven't crossed that bridge yet.

Originally Posted by Tony B

I am no expert but here is my take on this anyway.
Lets start off with fuel economy. Your boat is 27.5 ft long. Without knowing your length of waterline, I will guess it at about 23 ft.
Theoretical hull speed = 1.34 X the sq. root of your waterline length.
The sq. root of 23 Ft = 4.8
So, theoretical hull speed = 1.34 X 4.8 = 6.4 MPH
Your most fuel efficient speed other than just Put-Put will be slightly under 6.4 kts....So, your most economical speed will be around 6.2 Kts. Anything faster than that will greatly increase your fuel consumption with very little increse in speed.
As for the most stability in rough seas when the boat is on plane is true - within reason. It can get too rough to plane at times. Thats a matter of trial and error. Keep in mind that the truism of this is because the boat was designed for a higher speed.
I have the Westerbeke 4107 in my sailboat - approx 22,000 lbs. and I can go 6.5 to 7.0 kts on 3/4 gal per hour. That's over 8 MPG. I suspect that you will do at least that well. What you lack in hull shape you make up for in lighter weight.
Overall, if your main objective is fuel consumption and you like cruising at a slow speed, as I do, I think you made a wise decision.

The waterline length is actually 25 feet, and the centerline hull length is a tad above 28 feet. Hull speed is actually 6.7 knots. I would need roughly 20 horsepower to attain hull speed. I have an excess 10 horsepower to use to get the boat up to 8 knots if needed, and reserve for up to 12 knots. This engine is capable of running at wide open throttle for 60 minutes, but I am far from comfortable taking it there unless there is an extreme emergency.

I'm quite impressed the 4.107 can take a 22,000 lbs. boat to 6.5 to 7 kts! To be honest, I'm more than impressed! I have the proper gear ratio and propeller to achieve maximum efficiency based on this engine's capabilities, so I have little doubt the calculations will fail me in any significant manner.

Thanks, Tony! Your feedback has been very helpful...not to mention encouraging!

[Cheechako]

you definitely dont need 20 hp to get to hull speed on that boat. I built a 31 foot sailboat more than twice your weight and a whole lot more wetted area. easily did hull speed with a 13 hp volvo....