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Author Topic:   porposing sl-16
CHRISWEIGHT posted 10-13-2003 06:01 PM ET (US)   Profile for CHRISWEIGHT   Send Email to CHRISWEIGHT  
reading the various opinions regarding the sl-16 I am tending to find that the boat is not tremendously affected by trim but tends to porposing at very little provacation. triming in seems to help some but I am know worried that when I finally get her to sea she may become a very unruly boat. as a disabled boater (ex biker) I really need this aspect under control.

boat: SL-16 a little extra weight from rear A frame
engine: classic merc 90
prop :lazer 2 22pitch

any advice would be more than welcome.
chris

bobgilsp posted 10-14-2003 01:50 PM ET (US)     Profile for bobgilsp  Send Email to bobgilsp     
Chris-
The SL16 shouldn't porpoise until you trim it out so far that steering becomes difficult. When you are near top speed & trimmed such that steering effort is the same both port & starboard, you should have no porpoising. Check the height of your anti-ventilation plate. (See the postings discussed on this forum.) The anti-ventilation plate should be about one inch or a little more above the very bottom of the hull (above the bottom of the "V".)
CHRISWEIGHT posted 10-14-2003 08:23 PM ET (US)     Profile for CHRISWEIGHT  Send Email to CHRISWEIGHT     
ok at the moment the boat is not really well set up the trim fin on the mercury 90 was set dead straight when we picked the boat up,which gave very easy right turns but high effort was required to turn left. I have know offset the fin a small amount and she still turns right more easily than left but to a lesser extent, i will therefore try a little more.

the engine is in the second set of bolt holes, with the AV plate 1.5 to 2" above the bottom of the transom, no porpoising at or nearing WOT but at slower speeds around 25mph 3750rpm a small set of waves will seem to set it off.

This is my second boat and I suppose I am finding this action rather unsettling as my first boat (12 years) built in the uk and designed in Norway a Bonwico 300 by co-incidence a double skined foam filled planning fast fishing design, never porpoised. (it was also very small 12'6") hence the change to the Boston.

I suppose my real concern, is will this problem be amplfied in rough sea conditions.
chris

JohnJ80 posted 10-14-2003 08:47 PM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
Rough see conditions may actually help it since they can disrupt the oscillation. My personal, qualitative, belief is that the Dauntless is proe to porpoising. The same qualities that make it go so dry and well through a seaway also lend it the propensity to porpoise.

The answer to this is as follows:

1. Check your motor height again. If you are on the second set of holes, I think it is doubtful that you are up as high as you say. Usually, from the boats I have looked at 1.5" up is usually in the highest hole. The way to measure this is to position a straight edge along the bottom of the boat and measure when your motor is trimmed straight up and down. Note that many new motors allow some degree of negative trim - i.e. the motor tucked in more than vertical. Even doing it this way is hard to measure though. The right way to figure this out is experimentally, moving the motor up (or down, but usually up) until the ride improves and you are still able to have cooling water and no prop blowout.

2. You may need to add a fin to the motor. I would recommend the Grand Island Marine Turbo Lift since you don't have to drill the AV plate. This is the fin that whaler recommended to me.

3. move weight around and see what happens. Usually porpoising is a result of too much aft weight, although it can be forward weight too.

I have a Dauntless 15 and had porpoising in the ranges that you mentioned. I fixed it by doing the fin, moving the motor all the way up (and verifying cooling) and adding a high tech prop (Stiletto). Huge differences. Since I got so interested in boat trim, I also added trim tabs, which I have really come to love.

The tightness in your steering can also be related to the trim of your motor. As I trim my motor out, I reach a point (varies with speed) at which the the steering really becomes very easy and neutral. If I go to far, then it gets harder again. I think there is a sweet spot that is related to trim. If you have trimmed out so far that it gets hard again, I think you are out too far. The Dauntless will, absolutely, want to porpoise with too much up trim.

go to http://continuouswave.com/htdig and do a search on porpoising. There is a huge volume of data on this subject. I have a long thread on my repowering that also led me to solve a porpoising problem too. You can solve this, but it will take some experimentation.

Once you solve this, you will find this boat to be a very sweet and seaworthy craft. I love my Dauntless.

J

David Jenkins posted 10-14-2003 09:34 PM ET (US)     Profile for David Jenkins  Send Email to David Jenkins     
Is it possible that you have a pin installed that is not allowing you to lower the engine as low as it needs to go to push the boat with porpoising?
Moe posted 10-14-2003 10:00 PM ET (US)     Profile for Moe  Send Email to Moe     
If it steers easy to the right and hard to the left, with the trim tab centered, the motor is trimmed too far in (down), and the prop shaft is down at the rear (relative to the surface), with the prop thrust aimed down, trying to raise the stern and keep the bow down.

I suspect you might be having to run it that way to fight the prop, which with the Laser II is designed "for greater bow lift" according to Mercury.

Just a thought...
--
Moe

JohnJ80 posted 10-16-2003 12:10 AM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
What you don' t want in a Dauntless is Bow lift! Great suggestion!

I would look at the Stiletto props. I have great luck with mine and they help the porpoising problem. Call the prop mfg and talk this through with them.

j

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