Pearson 30 atomic 4 optimal rpm at hull speed?

[tkeithlu]

Scuba seas, I'm afraid that you have overstated a good point. Fuel consumption relative to speed is exponential above hull speed, but there are non-planing sport fishing boats out there that trade massive power for speeds well over what their hull lengths would dictate. As the fighter jocks say, give me enough power and a barn door will fly.

Hull speed is simply that last point at which the stern is supported by the return of the bow wave. After that, a hull has to climb the bow wave, literally with its bow up and its stern down. Even a planing hull does that; it's called plowing, and occurs before the excess power lifts the boat up onto its bow wave, aka planing.

[Scubaseas]

Originally Posted by tkeithlu

Scuba seas, I'm afraid that you have overstated a good point. Fuel consumption relative to speed is exponential above hull speed, but there are non-planing sport fishing boats out there that trade massive power for speeds well over what their hull lengths would dictate. As the fighter jocks say, give me enough power and a barn door will fly.

Hull speed is simply that last point at which the stern is supported by the return of the bow wave. After that, a hull has to climb the bow wave, literally with its bow up and its stern down. Even a planing hull does that; it's called plowing, and occurs before the excess power lifts the boat up onto its bow wave, aka planing.

I agree but what I said was "You can not get a displacement hull to go faster than the hull speed which is determined by water line length." I am not talking about a planing hull or a semi displacement hull. A Pearson 30 is definitely not a semi-planing or planing hull. If the OP is thinking to toss enough power or prop at it and make it go faster it ain't gonna happen.

[thruska]

Originally Posted by guyrj33

I’m referring to speed through the water, speed over ground has nothing to with hull speed discussions.
12 kt in a 10m is very exciting.

Ive gotten 12 knots with Columbia 28 on Long Island Sound, it didn’t submarine...
And 9 knots on ICW in good tide and current.
Thats with a 69 A4. Yeah its direct drive but it’ll slip if its not adjusted correctly so is that direct ? Really ?
Its got a carbon throw out bearing like a mg midget.
Adjust it correctly and i could get 5 knots in reverse.
3/4 trottle and you could do hull speed all day long.
My thermostat **** the bed so i put in a throttling valve for temp control. Also had a garden hose spigot for hot showers underway if you’ve a mind too. Worked best anchored..

Did you try unsticking the A4 ? Try an atf and diesel cocktail in each spark plug hole soaking a few days.

[Svsumurun]

Not that I really 'measured it' but my experience with the P30 that it was fast for what it was .. but under motor .. 6.5 knots, that was about it, so that's basically hull speed- and you really didn't have to push the engine, really the only time I was ever above 2000 rpm was when we went up the Detroit river against the current or in reverse stopping it when we came in too hot to a dock!
The speed of the boat was under sail - I never understood it , but 10 knots was not hard to hit, and pushing it we got over that several times ( but our sailing tends to be casual, if possible- so usually we'd back off if that happened)

[JimsCAL]

Something tells me there are some people here with poorly calibrated knotmeters.

[Svsumurun]

Originally Posted by JimsCAL

Something tells me there are some people here with poorly calibrated knotmeters.

I get what your saying Jim, and 'hard' numbers are not something I really worry about , but we all know that certain boats are faster than they should be ( under sail- prop power is what it is) , and some are slower.. I might even agree with your calibration statement, other then I replaced all the instruments on that boat from old stuff to brand new top line units and it was basically the same.
The P30 is an old design, and going up against modern 'fast' boats would get its lunch ate, but I can think of several clubs here in Lake Erie alone that still have P30's that are regularly club champions.... that was all my point was with the 10 knot speeds I quoted above- the boat , for some reason I really don't know (or care about) was crazy fast for what it was... unlike my current boat!
None of this matters to the OP .. he's trying to figure out what he needs with an electric setup to move the boat , and that will come down to where he is .. against current or tide , he'll have to have some power ( going up the Detroit river sucked and was slow with the A4) if he's not in that situation, then he doesn't need much .

[Scubaseas]

I am having a hard time imagining a boat with a D/L of 237 and a PHRF of 182 to 200 can manage 10 or 12 knots.

[Svsumurun]

A smarter guy than me could maybe answer this , but maybe the crazy rudder had something to do with it?
The 'bite' that you felt the boat have when she got a bone in her teeth through the tiller was a different feeling then any other boat I've ever helmed . On the other hand it was a total PIA sometimes .. it would try to turn backwards and rest under the boat at the dock or anchor , you were better off just turning it that way and lashing the tiller up

[JimsCAL]

I am very familiar with the P30. I've raced against them and their PHRF rating of about 180 is pretty fair. Lots of these out there for lots of years, so their capability is very well known. They will not do 10 knots through the water. Maybe a few second surge under spinnaker with perfect conditions, but that's about it.

[sailortype]

my previous boat was a P30, had it for 20 years. Although my instruments were ancient SR Mariner, I don't think they were too far off. Never flew a spinnaker but under white sails, I was flying at 6 knots; only once exceeded that at 7.8 knots surfing down the back of a wave. Cannot believe 10 knots at any time.

[LakeSuperior]

Originally Posted by Scubaseas

I agree but what I said was "You can not get a displacement hull to go faster than the hull speed which is determined by water line length." I am not talking about a planing hull or a semi displacement hull. A Pearson 30 is definitely not a semi-planing or planing hull. If the OP is thinking to toss enough power or prop at it and make it go faster it ain't gonna happen.

This statement is just not correct for a displacement hull or any hull, sorry! No physics support your statement. Go back to your Wiki page and read it. Hull speed is an obsolete concept. It is simply where the power versus speed curves gets steep.

[NorthCoastJoe]

I have a Pearson 30 with a Yanmar 3GM30F with a matched prop. I never seen 10 knots under power or sail and yes I have more power than I need.

[NorthCoastJoe]

If you want to go diesel I have a Yanmar 2GM that I took out of my boat. It has a fresh valve job and cooling jacket cleaning.

[guyrj33]

Originally Posted by LakeSuperior

This statement is just not correct for a displacement hull or any hull, sorry! No physics support your statement. Go back to your Wiki page and read it. Hull speed is an obsolete concept. It is simply where the power versus speed curves gets steep.

The day I saw 12 knots through the water while single handing my Pearson 10M was a special day, but also the perfect day to illustrate that displacement hull speed is a theoretical number and not an absolute maximum.
I was traveling North entering Gloucester Harbor under power with bare poles, there was a 40 knot wind behind me and I was surfing down the face of some pretty big waves 10-20ft coming in from a storm out in the N. Atlantic. Big smooth rollers finally hitting shallower water. Speed through the water was 12 knots for sustained amounts of time.
In perfect conditions i've exceeded my theoretical hull speed of 7.2 knots under sail alone, but only by a knot or less. Broad reaching with lots of sail up.

[Scubaseas]

Originally Posted by LakeSuperior

This statement is just not correct for a displacement hull or any hull, sorry! No physics support your statement. Go back to your Wiki page and read it. Hull speed is an obsolete concept. It is simply where the power versus speed curves gets steep.

OK fine, You're right, I'm wrong. I don't disagree with you or tkiethlu.



For practical purposes at what point do you get a displacement hull to plane? You have NorthCoastJoe with a 6.5HP/ton power plant (27 HP in a 8300lb boat) and he is not exceeding hull speed by much if at all. If you put a 40 HP motor in a boat that needs 18 HP to make it run it at hull speed all you will do is make the stern squat, you're not going to get it to go faster than hull speed. I would bet more than a few cold beers to guess if you tossed 80 HP at it you still could not get a P30 to get up on a plane. I get the whole climbing the bow wave thing.



Feel free to disprove the theory that some clipper ships were lost due to over powering and pushing them down.



Please tell me what happens if you try to tow a displacement hull like a P30 at 15 knots or more? Typically the stern sinks and the boat can downflood. Show me where it's OK or normal practice to tow any sailboat faster than it's hull speed?