Reverse strakes, planing chines

[skypoke]

I have a 28' power boat which needs a bit of lift. Problem is, it seems to not want to achieve plane, or for that matter stay on plane, without an excess of throttle. This is a monohedron, moderate v hull. I have trim tabs which help somewhat. There are no options for reducing weight.

A friend with a large Hatteras was faced with the same problem. He had some (please excuse my ignorance, I'll call em reverse chines) installed along the outer chine of the hull. These protrude out about six inches, run approx the rear 2/3 of the hull, taper forward and have a slight concave curvature as viewed from below. They added several knots to his speed, increased pounding a bit and increased stability at rest. If you look at a Bertram 31 you'll see something similar, albeit smaller.

I'm thinking of fabricating and adding. I was wondering, however, how to calculate an appropriate area for these, if there is a standard factor for lift on plane per sq. inch of surface normal to the water's surface. Is lift proportional to speed or is planing a magical region where speed relationships are exempt?

Would appreciate your general opinions on the concept, I'm not seeing other options for adding planing surface. While I hope this is the magic bullet, at the worst if it doesn't work out I'll just grind em off... Thanks, Chuck

[gonzo]

What is the horsepower and weight of the boat? Adding more bottom surface won't help without the extra power to increase lift. From what you are saying, the need for exesive throtlle, there may be other things to look at. If the power/weight ratio is correct, the propeller pitch and diameter may be wrong. Also, what is the transmission or outdrive gear ratio?

[skypoke]

GONZO,

Engine hp...this is a JT 671 TI with a theoretical horsepower of 460 @ 2500 rpm. However, due to fuel consumption, noise, I like to run it more in the 1800 RPM range. At that speed the turbo is spooled and the engine seems happy.

Gear, twin disc 507 1.5:1

Prop, originally 24 X 24, now 20 X 24, was overloaded on engine with larger pitch...black smoke, heat. Prop angle is a bit high, probably, engine is mounted at a 14 degree angle to stringers, gear is straight, no down angle built into gear. Has a pretty draggy, probably, strut below to support the 2" shaft but nothing out of the ordinary.

Weight is tough...this is a project I joined in on with a buddy, kind of hitched my wagon to his star, I guess. This boat was originally a John Allmand hardtop model. We stripped it to bare hull shell and built from stringers up. It has ply bulkheads, 1/2" glass over ply sole, bare cuddy. It's built pretty light compared to original, the hardtop we removed had to weigh 1000 lbs, inner liner with lots of 3/4" ply w. heavy glass, windshields, accommodations,hundreds of pounds of putty hit the dumpster as did the twin gas engines, shafts, gears. It now is basically an open boat with a lightweight aluminum frame hardtop and small cuddy. It carries 200 gal of fuel in below deck side tanks. Engine weight is 2600 lbs with gear, shaft. Don't know the weight of the twins we removed, but would expect that all up weight of orig power was around 1800. Boat floated a bit higher than original lines, balance about right, running trim is good. Eyeballing, engine is mounted very close to center of flotation.

Running it, though, it seems to me that it is wanting to operate in some kind of semi displacement mode. I know, compared to other boats, that the hp should be sufficient for what we have.

No need to state the obvious...my next boat is now at the plasma cutter's, a product of meticulous design by a well known designer! But I do need to get this one to a decent level of performance, somehow.

Thanks, Chuck

[Raykenn]

Skypoke,

You mentioned that reducing weight is not an option. When underway does the boat tend to trim heavily by the stern? Is shifting the locations of the weight an option? Are the fuel tanks in their orgininal locations or were they shifted during the "renovation".

Regards

Ray

[skypoke]

Raykenn,

The fuel tanks are aft of the original location, one port one s. The engine is mounted forward (it's a single engine) of where the dual engines were. Running trim is not bad, certainly not the level of squat I see in most inboard sportfish boats. When on plane, at the transom, the top chine is immersed approx 12".

The balance of the boat when at rest, and loaded with fuel is very close to correct, fore and aft. When light on fuel, at rest, it's a bit bow heavy. It throws a significant wake to my way of thinking this is related to excessive immersion of the hull while underway.

Chuck

[Tom Lathrop]

Sky,

If this boat is a moderate deadrise monohedron and there is nothing dreadfully wrong with the underwater lines or weight balance, then its ability to get on plane can be reduced to two factors. These are power to weight ratio and bottom loading factor in lbs per sq ft of bottom planing area.

In such a simple hull shape, you should be able to get a good estimate of the displacement from the immersed volume at the waterline. Likewise, the planing area of the bottom should be fairly simple to figure as 65% of the at-rest area.

Many NA's say that power to weight ratio should be 50 lbs per HP or below for good planing. Some advise that 25 would be much better. I find that on a well shaped bottom with a low bottom loading of less than 40 lbs per sq ft, reliable planing can be had at well over 50 lbs per HP.

Stands to reason that high bottom loading calls for low weight per HP. Similarly high weight per HP calls for low bottom loading. You can trade one for the other but if both are at the wrong end of the scale, planing will be difficult.

Some for of lifting chines like you mentioned may help get you onto plane even better than trim tabs. I would use them on moderate speed boats (less than 30mph) to aid early planing but not for high speed. At high speed (don't know how high) they should tend to depress the bow too much and might also cause chine tripping in high speed turns.

Work out these numbers and let us know the result.

[skypoke]

Hmmm,

Makes sense. I will need to do some forensic calculations for weight, I have the material list that went into the boat so I can figure this, know drivetrain weight. The one thing I don't have is weight of hull shell itself, I'd always guessed at this as approx 3500 lbs, heavy laminate, no core. Guess I can figure the square footage of hull based on an average thickness of 3/8".

65% eh? I'd always wondered what the relationship between an angled hull and upward lift was. It'll be easy to figure the footprint of hull in water, essentially a rectangle and a triangle.

I'll put the figures together and let you know what comes up.

I really appreciate the help. It sounds like this problem will be demystified at last. Should have the figures in a couple days.

Chuck

[Tom Lathrop]

Sky,

Rather than try to guess the weight of all the stuff in the boat, I meant for you to estimate the displaced water by the hull.

If you know or can measure the priciple dimensions of the hull and its draft, you can get a fairly accurate estimate of its displacement in cubic feet and multiply by the weight of water at 62.5 (fresh) or 64 (salt) lbs per cu ft.

[Reese]

Here is an interesting product called speed rails that act very similar to reverse chines. I cannot personally vouch for the any of the claims made by the manufacturer, and although a few member of another forum have tried them it was on light 1,500 lake boats that go about 80 mph.

This product does not work very well on light fast boats and is much better suited for large slower boats that need help planing.

Good luck and let us know how they work if you try them, here is the link.

Speed Rails

[Tom Lathrop]

I saw an article on these when they first came out. They are intended for fast boats not slow ones. The wings on the aft sheer are also claimed to give aero lift which would only be effective at high speed. They claimed to have a patent on the rails but they were used and called a wave collector almost 100 years ago! See Weston Farmer's book "From My Old Boatshop". An Australian got a patent on a similar system called the CAT (Controlled Air Technique) at about the same time as these Swedes. Both patents are suspect since the ideas were already in use for a long time. Similar rails have been used on boats that run races on the Yukon River in Alaska every year. There is very little that is really new in this field. The website on Speed Rails was probably written by a marketing man.

Do they work? Probably there is some benefit. I am sure that my chines, which are much larger, also have a transverse down angle for spray control and lift, help get and hold my boat on plane at a lower speed than otherwise.

[gonefishing]

I have a similar problem. My hull is 25' and heavy for it's size, about 8,500 lbs. With twin gas inboard she was slow getting out of the water. With new diesels, that is no longer a problem. Now is more an issue of stability. She is a little tender but runs fine up to about 22 kts. So long as you don't get carried away with the tabs you're fine. By that I mean to hold the tab control for just a couple seconds and wait for the boat to react before moving the tabs again.
The problem now is one of stability at speeds between 25 and 30 kts. The hull tends to tilt to one side. You can level it with the tabs but its not easy. The hull "tenderness" is amplified at those speeds. She's so tender it is very easy to over do it and end up tiliting to the other side.
I've been looking for ways to stabilize and level the hull without the use of tabs. It's really a shame (or waste) to have the power of the diesels and not being able to utilize it. At 22 kts the turbos are not even whining. Any suggestions for modifications/additions or references of some architect that can help would be appreciated.

[gonzo]

That is a problem of dynamic instability. The flow of water under the hull is creating dynamic pressure with the wrong distribution. You need someone to look at the boat, measure and calculate loads, lift rails if any and general underwater shape. It is not a simple problem.

Originally posted by Tom Lathrop
...The wings on the aft sheer are also claimed to give aero lift which would only be effective at high speed...

I especially liked their discussion of aerodynamic drag: "Do not the wings create drag? Yes they do, however at this relatively low speed is it of little importance. " If the speed is so low the drag is of little importance, then the lift is of little importance, too! The wings are pure styling.

[rkpshenoy]

Can the chine line be totally immersed in water throughout the LWL of the demihull.
If so what will be the resulting planing characteristics.