<< Subj: Re: Fwd: Hawaiian trade wind P14/15 HP race
Date: 98-11-09 02:19:58 EST
From: kent@songbird.com (Kent Crispin)
To: wwpotter@tscnet.com
- - - - - - - - - - - - - - - - - - - - - - - - - - -
On Thu, Nov 05, 1998 at 06:28:04PM -0500, GSTahoe@aol.com wrote:
> Rich--
>
> With a P-15 in 20 knot winds with attendant chop if you where to race three
> boats, one with a Honda 2, One with a Tohatsu 3.5 and one with a Honda 5
from
> island to island, I'm afraid it would be a dead heat. All three would push
> your P-15 at hull speed.
I don't think this is true in general, is it? The thrust generated by
the motor is a function of the HP, and a larger HP motor will
generate more thrust. On the other hand, the resistance to forward
progress comes from two factors, at least: the resistance of pushing
the hull through the water, and the resistance of pushing the part of
the boat above water through the air. The thrust of the motor must work
against *both* of these factors.
That is, if you have a motor just powerful enough to move you at
hull speed in still air, you will go slower than hull speed with a
head wind. Contrariwise, a weaker motor will still drive you at
hull speed if you have a tail wind adding forward thrust.
kent
--
Kent Crispin, PAB Chair "No reason to get excited",
kent@songbird.com the thief he kindly spoke...
PGP fingerprint: B1 8B 72 ED 55 21 5E 44 61 F4 58 0F 72 10 65 55
http://songbird.com/kent/pgp_key.html
>>
We've shot down this concrete wall Maximum Efficient Hull Speed absolute
theory before...
I quote a previous response to this crab crusher absolute Maximum Efficient
Hull Speed rule designed for freighters and tankers a millenia ago...
"However, that resistance (climbing the bow wave) isn't the only thing that
limits a boat's speed. If the headwind presents so much resistance that the
bow wave climb isn't reached because the motor can't go that fast (not enough
thrust), then the hull speed isn't the issue.
However, I can easily imagine conditions where 2hp wouldn't push a big bluff
bow like the P15 has at hull speed at all.
As a final comment, if it takes, say, 3hp to reach hull speed under a given
condition, then a 5 hp engine can put out 3 hp all day long a lot easier than
a 3.5 hp engine run nearly flat out. So if you think you're going to need to
power the boat for long distances, a motor that's a little overrated won't
hurt, providing the weight isn't an excessive issue.
---------------------------------------------------------------------
It is a fact that the 2, 2.5, 3, 3.3 and 3.5 HP are the same engine with
different carb, gear ratios and weight. The motors over 3HP have neutral and
forward an excellent feature when you are trying to tie up to a dock... The 2
and 2.5 have fwd only. You can't stop the darn prop from turning and pushing
you where you don't wanna go without shutting off the engine...
Yes in flat seas and windless conditions all 3 motors would push the boat to
the maximum efficient hull speed. But the 3.5 and 5 are able to push it a
little bit higher onto the bow wave and a lot faster against wind and wave
action...
...
The concrete wall theory (MEHS) is wrong. It has to do with type of vessel,
displacement to hull ratio, wave action and wind speed. It has be severely
violated by all the new ULDB vessels available. Enough said, a vessel like
the Potter 15/19 should be able to exceed Hull Speed. I would suggest you go
back in the postings and read the previous treatise as I am unable to copy
it.
A 3.5 will push the boat to hull speed in adverse wind/sea conditions when the
2HP will be gasping to maintain headway or even falling back... and that is
without the problem of not having a neutral to shift to, when slowing down to
tie to a dock...