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Chris c 03-14-2012 10:55 AM

Extending inner strakes on 38
 
Wondering if anybody has extended the inner strakes on a 38 Cig.If you had what pros and cons have you noticed. I have a 84 flat deck and i measured the strakes on a late model 90 s top gun and they appear to be a foot longer.
Thanks Chris

kvogt 03-14-2012 01:32 PM

90 Top Gun has a notched transom, maybe that has someting to do with it? I'd try alot of different props before even thinking of extending the strakes? What does the flat deck do wrong that the 90 Top gun does better running wise?

c_deezy 03-14-2012 01:46 PM

There was a member on here that did a write-up a few years ago on how to temporarily extend your lifting strakes for testing before fully committing. I'm pretty sure his screenname is 'BenPerfected'.

bigfarmer 03-14-2012 04:00 PM

I' believe if the strakes were longer it would add more lift. I'd think the longer they are the more stern lift it would create. Maybe Gary (RMPRam) can chine in I think he has or had some lifting strakes that were going to be mounted on his flat deck. Don't quote me on this as I'm not a professional and just a poor dirt Farmer:lolhit:

Chris c 03-14-2012 05:36 PM

Thats what i am thinking. I have alot of weight in the back of the boat. It has psi intercooled blower motors along with #5 s on boxes. I know the boat is 28 years old.

MIskier 03-14-2012 06:47 PM


Originally Posted by bigfarmer (Post 3641267)
I' believe if the strakes were longer it would add more lift. I'd think the longer they are the more stern lift it would create. Maybe Gary (RMPRam) can chine in I think he has or had some lifting strakes that were going to be mounted on his flat deck. Don't quote me on this as I'm not a professional and just a poor dirt Farmer:lolhit:

That is a common misconception, that generally is not true. Due to the direction that the fluid moves across the hull bottom as you move further aft the lifting strakes become less efficient to the point that they only add drag.

I have a paper somewhere that has a nice illustration of this flow. If I can find it I will post the illustration.

bigfarmer 03-14-2012 06:58 PM


Originally Posted by Chris c (Post 3641288)
Thats what i am thinking. I have alot of weight in the back of the boat. It has psi intercooled blower motors along with #5 s on boxes. I know the boat is 28 years old.

What are you running for props? 5blades? Spinning which direction? I was told to 1. Run the prop shaft 2.5" above the running surface of the boat. 2. Run 5 blade props spinning in. 3. Add a wedge to the back of the boat. All 3 have to be done or its just a waste of time trying to run this recipe. What hp are you running? And what's your top speed? Roughly 850hp has got me to 97mph. Thats 4 blades with the prop shafts level with the hull. In the works of 5 blades, raising prop shafts, and the bottom mod.

automobili_v12 03-15-2012 05:38 PM

It would probably make it faster, it will rise the stern out of the water reducing drag from the rest of the V Bottom. Keep us posted on anything you find. I got a couple of projects im working on and I will probably play with the bottom also.

tcelano 03-15-2012 05:58 PM


Originally Posted by MIskier (Post 3641336)
That is a common misconception, that generally is not true. Due to the direction that the fluid moves across the hull bottom as you move further aft the lifting strakes become less efficient to the point that they only add drag.

I have a paper somewhere that has a nice illustration of this flow. If I can find it I will post the illustration.

Agreed. Most of the lift of a planing boat is generated around the stagnation point, or where the water first hits the hull. The pressure spikes there, and then trails off pretty rapidly. That's why you can run a boat with a CG 10-11' forward of the transom, and have only 14-15' of the hull in the water.

Think of it this way, to support (round numbers here) a 10,000 lb boat and you are making it sit on a section of hull 8' wide by 10' long, then your average pressure would only need to be 0.86 psi. Realizing that at 70 mph, full stagnation pressure is around 70 psi, and your hull is running an angle of attack of let's say 3 degrees, you have about 9 psi probably around where the spray root is hitting.

How much area then, does it take to support the boat? 10,000 lb/9 psi = 1110 in^2 = 7.7 ft^2. So that confirms the hypothesis that the majority of the hull lift is probably coming from probably the first foot or so of running length.

So, messing with your running strakes far aft will likely do very little for you. The place where they are terminated is much more likely to either be completely arbritrary, or chosen so as not to disturb the flow into the props. I'd spend more time on something else, unless you have the empirical experience of a builder that has literally tried thousands of variations.

CignificantOther 03-16-2012 11:31 AM

With the larger motors and drives, you have moved the COG further back than the originally designed TRS hulls. Additionally, you have increased the speed well beyond the average Flat Deck from the day. You are are also probably losing some leverage with the higher drive height. All of this "may" be effecting lift, etc, thereby changing the designed attitude of the hull. Extending the strakes may compensate for your weight/speed modifications.

Years ago I suggested extending the lifting strakes on a friend's 24' P & D about 16". The drive had previously been raised, which in my opinion was reducing bow lift. The boat responded well, with more bow lift and about 6 mph. I've thought about doing it for several years on my Gun, but never wanted to grind on the original gel.

I say....try it.


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