Bottom strakes

[Boysie]

Hi folks,
A number of amateur builders are mounting two triangular strakes each side of the centreline on the bottom of their planing hulls. The strakes are cut on the 45 degrees from a piece of 2X2 plank and mounted with the triangle pointing downwards to some degree depending on the deadrise, eg the bottom of the strakes do not lie horizontally.
Whilst they probably do not provide lift, it is claimed that they assist holding the boat on course when running slow.
Having no rudder, jet boats are notorious for wandering around at slow speed and I'm wondering if these strakes were cut precisely so that the triangle pointed exactly downwards would help. Also, would they be detrimental to planing speed?

I have found on the planning boats I built that strakes can help directional stability to a point . A pair about one third of beam out and SHORT, from Bow aft about 2.4 m, is all thats needed
i have found that when we added full length and two that the boat ran HARD, with no added benefit to tracking
from Horizontal to dropping say 3 mm, over a 50mm distance
When I see boats covered in these things I wonder what on earth for all they achieve it seems to me is a hard ride

[Jango]

I believe Strakes were added to Help achive the beneficial effects of a concave forward Bottom Not easily posible with either a molded or plywood Hull. Properly designed, Strakes, should have little effect on Planning Speed.

[Boysie]

Thanks for replies.
Lazeyjack, so for a 19 ft boat one strake each side running parallel to the centreline, presumably starting at the waterline at rest and running back 2.4 metres. Would the outside face of the strakes be vertical and their bottoms run 3 degrees up from horizontal towards the centreline?
Alternatively, what if I made them triangular with their outside faces vertical? I've read on this forum that flat strakes effectively reduce deadrise and I don't want to increase pounding.
You'll be warmer than we are at present. Regards.

OK LIKE THIS. if you take the angle of bottom(deadrise) at various places along the bottom, then you can draw the strake,a s you can see, the strake narrows as the deadrise steepens This is very rough sketch, I am hopeless in rhino, No you dont start near wl, but around thrird beam out from cl, will try dig up some pics or one of the guys who know CAD can do a better job for you Yes been 24 c for ages but last two days drizzle and 15-17

[marshmat]

Hi boysie,

Since you're talking jetboat, the first rule of strakes is to keep them well away from the intake and never in front of it. Strakes ahead of the intake will do nasty things to the continuous, undisturbed flow the pump needs.

If you have a relatively shallow-V boat, adding strakes might help with directional stability at planing speeds. I doubt you'd see much difference at displacement or transitional speeds, although you may find the boat breaks over the 'hump' a tiny bit earlier. Most planing-hull boats designed for very high speeds flat out suck at low speeds anyway! If you have a directional stability issue, the solution is more lateral resistance aft of the pivot point. High performance pumps (take a look at the Aggressor 13000 or Berkeley 12-series for example) often have a small fin hanging below the steering nozzle for exactly this reason- the added lateral plane greatly improves directional stability, both in straight lines and when turning at high speed.

Strakes are more commonly seen on deeper V forms, where they can help to alleviate 'chine walking' and can improve planing efficiency slightly.

There are few hard-and-fast rules and little freely available scientific data regarding good strake configurations. Whenever a designer or builder finds one that works, all the test and development data is quickly stamped "confidential" and hidden in a locked filing cabinet.

[Boysie]

I've just looked at those on their websites. Interesting. Are the fins retractable?

[Shorebreak]

Boysie,

As mentioned previously, strakes are good for high speed, not low speed. They create a definitive separation point for the flow, and show merit when the flow is trying to separate from the hull bottom before it gets to the chine (chine walking may be an extreme result of this situation). My rule of thumb is to locate them to achieve an 80-85% effective beam when the flow is separating at the strakes instead of the chine, but depends on the top speed. At volumetric Froude numbers below 4.0 (for top speeds) I do not use strakes. Width and angle of the strake surface will also depend on other design parameters, so consult a naval architect before building them.

Using fins with waterjets is a must with a low deadrise boat, also previously mentioned. High aspect ratio fins are going to have less drag, but low aspect ratio fins are going to minimize draft. I've never seen retractable fins, but expect that it isn't out of the question. Again, consult a naval architect before proceeding with construction.

Darron

[Willallison]

There are few hard-and-fast rules and little freely available scientific data regarding good strake configurations

Matt's absolutely on the money with this last statement - there are as many different spray rail configuration's as there are designers - and each will tell you that their layout works the best!
There are a few general "guidelines" you should observe though.
Spray rails should be horizontal on the bottom surface or angled down slightly - but no more than 2 - 3 degrees. If you keep maximum chine width down to around 2 - 3 % of maximum waterline beam then they shoudn't have a detrimental effect on ride quality.
The transition from bottom to rail can be smooth, but it's outer edge should be as sharp as possible, as should the chine.
The primary purpose of a spray rail - as its name suggests - is to separate spray from the hull, reducing the wetted surface area and thus drag. They will also have a beneficial effect when it comes to directional stability, though I agree with Matt that you are unlikely to see much advantage at low speed.
Most would agree that higher speed boats benefit from 3 rails, slower boats from 2 or even 1. The outer-most rails should be the longest and they should become progressively shorter as they near the keel.

[Boysie]

Thanks for replies guys.
I will definitely not be using strakes. My concern about raising the fin was if it protruded below the bottom it could be damaged if the boat ran aground.
I am not aware of any locally manufactured jet units with fins, so what do you think of this idea: Jet skis have an oblong plate maybe 12 inches long and 8 inches deep bolted vertically on each side of the hull near the stern. The plates have a series of vertically arranged mounting holes to adjust the amount they (the plates) protrude below the chine. (Sort of like low aspect ratio lee boards.) My boat is going to be used for sea fishing and my main concern is to keep the boat running straight at slow speed when hauling a longline and when coming alongside a dock.
Is this feasible? For a 19 foot boat 7ft 6ins across the chines and a deadrise of 10 degrees what might the dimensions of the plates protruding below the chine be as a starting point?. From there I could make up a series of plywood patterns to see what shape and size works best. Regards,.

[Timm]

My own thinking follows closely what Darron suggested. I tend to use them for spray control, so I don't put them too close to the keel for fear of breaking the water off the bottom too far forward and making the boat wet. I'd rather the water come off cleanly further back, hopefully the spray will be past you when the wind grabs it and throws it back! I generally angle them down at about 5 degrees, I was taught 7 was the max.

[Shorebreak]

Quote:

There are few hard-and-fast rules and little freely available scientific data regarding good strake configurations


For Matt and Will,

I must disagree with your opinion... My design criteria is based on linearized data taken from a substantial volume of tow tank data performed for planing craft, which is all available to the public domain, at least in the U.S. For the wetted portion of the hull, each spray rail, when required, is carefully located through calculation to increase stability and/or minimize drag due to whisker spray.

Boysie,

Sounds like you've come to a good conclusion. Good luck with your project.


Darron
Shorebreak, LLC

[Willallison]

Darron
I've no doubt your methods result in an efficient result. What I was suggesting, however, is that there are as many 'design methods' for the size and placement of spray rails as there are designer's; and each will insist that their's is the best.....
Having said that, I'd certainly be interested in taking a look at the information to which you refer, if you're happy to point the way towards it....

[tom28571]

Missing so far from this conversation is the reason these spray rails are used in the first place. They are primarily an attempt to lessen some disadvantages of the deep V hulls like slowness to plane and high power requirements. The resulting hull tries to include advantages of both deep V and shallow V in one design. They do this by adding some flat or nearly flat planing area to the deep V hull bottom which is an inherently poor shape for planing or efficiency. Number and width of strakes, placement and angle can also increase stability underway such as a reduced tendency to chine walk.

[Willallison]

Tom
I think this may be a 1st... I fear I must disagree with you! I consider - and would reckon that many more eminent than I would suggest - that the primary function of spray rails is to separate the water flow from the hull, thus reducing wetted surface.
I agree that they may have a small impact on the lift generated, particularly in a high-deadrise hull, but this would surely be a less significant aspect of their impact...