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Subject: Re: We're in a quandary


Author:
Robin Carter
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Date Posted: 11:14:32 12/01/2009 Mon
In reply to: Alan Smith 's message, "Re: We're in a quandary" on 08:53:53 11/01/2009 Sun

>Thanks Ian. I'll get the prop size to you later today.
>
>Alan

Hi Alan
Just to muddy the water there are more factors surrounding the speed of your boat, how you measure your speed is first? is it by a log over water? if so that's at best a very approximate guess, my Nasa log read 20% low out of the box. A gps plotter will be acurate, and show true speed over land, if gps what is the effect of tide. What revs are you using at 6 mph? my Perkins book is on the boat (its raining and I don't want to get my car dirty so its staying there for now!)but unless you have a bundle of revs I'm only guessing but I suspect you are unlikely to be getting 40 HP.
Hull speed (see wikipedia - Hull Speed) calculates your boat at approximately 6.5 knot (6mph is just under 4.5)so in principle you may get another couple of knot, thats if you're not (sorry knot intended to pun!)already getting it and your log is lieing, very likely! if you don't have a gps or plotter you need to borrow one and check.
Hull, Props, engine and gearbox matching is theretically a science but in practice its almost a black art, there is a lot of b~#@+**ks talked about it (most by so called experts),a well set up outfit will exceed its hull speed (not by a lot though), the best guide is to find a similar boat and compare the performance, as a start my 27 is similar ( a touch longer) has a Perkins 4.108 on a 1.1 box, 12 x 8 prop, and will max at 7.5 knot (gps measured) at 2200 revs (at those revs thats a lot less than 40hp) and I think at that its on its hull speed and doubt that it would do much more without a vast amount more horses.
I doubt a new engine will give you more speed unless your Perky is really gasping, I would look to checking what you are actually doing first, and then accurate set up i.e. correct prop in good condition after that, cheaper than a new engine. Now if you are primarily looking for reliability thats a different discussion.
Best regards
Robin (Sailbad the Sinner)

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Replies:
Subject Author Date
Re: We're in a quandaryAlan Smith15:55:27 12/01/2009 Mon


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