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Old 30-01-2016, 07:23   #1
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How much does crew weight affect cruising speed.

Given there are so many smart people here, and I'm too lazy to read a physics book or search for horsepower ratings (for a given LWL, Ballast, Displacement and Sail area), how much does the weight of additional crew affect the boat speed of a "typical" cruising boat?

My intuition tells me, not much.

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Old 30-01-2016, 08:06   #2
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Re: How much does crew weight affect cruising speed.

Good questions and I like your sense of humor...but what is "additional crew" and what is a "Typical cruising boat" ? So the NTSB and Coast Guard have regulations about such things for airplanes and passenger vessels, and over the years I believe that number of crew/passengers has gone down as average weight of Americans has gone up. A football team front line on a 30' boat will change waterline, draft and performance but probably not much on a 60' boat--but monohull ULDB or catamaran or heavy cruiser or what? Eight Japanese schoolgirls (and dont get funny on me here) might change little of anything--but dont tell your wife!. Most modern boats are designed and powered with weight of crew in mind and of course we have capacity ratings on boats too. I am sure that a physicist/engineer can help more (a lot more I am sure) but can you provide some more specifics about the boat and your plans and why this is such a concern? Maybe you will get more substantive answers. (And is this after or before lunch when they board?)
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Old 30-01-2016, 08:09   #3
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Re: How much does crew weight affect cruising speed.

Figure your mass per inch of immersion, that will let you know how much extra wetted surface area the "extra" crew is causing.
From this information you can extrapolate a "useful mass index".
Those that are below your arbitrary useful mass threshold can be ejected from the boat on the grounds of good solid reasoning backed up with some fuzzy math.


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Old 30-01-2016, 08:19   #4
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Re: How much does crew weight affect cruising speed.

I can feel the difference of an extra crewman on my 2800 pound 8.5 meter cat.

It is worse with 6 more and a cooler full of beer!
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Old 30-01-2016, 08:25   #5
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Re: How much does crew weight affect cruising speed.

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Originally Posted by Paul Annapolis View Post
but can you provide some more specifics about the boat and your plans and why this is such a concern? Maybe you will get more substantive answers. (And is this after or before lunch when they board?)
We race cruising boats (oxymoron though that may be) so the question is how much crew weight can a boat handle and still be competitive, relatively speaking. For example, would going from 6 crew to 8 or 10 on a 34-36' cruising boat so dramatically affect the performance that one needs to seriously consider trading up to a 40-41' boat to handle the additional weight, assuming crew comfort is not an issue.
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Old 30-01-2016, 08:30   #6
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Re: How much does crew weight affect cruising speed.

It depends on the wind strength and which side the crew are sitting more than any other calculations. Light wind - leave before all the crew arrive at the dock 🙈
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Old 30-01-2016, 08:32   #7
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Re: How much does crew weight affect cruising speed.

Depends on the boat, of course.

Affects some more than others.
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Old 30-01-2016, 16:23   #8
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Re: How much does crew weight affect cruising speed.

Figure that each crew member with gear weighs about the same as another 100 litres of fuel or water in your tanks.

On my cat, an extra half a dozen people makes no noticeable difference.

Racing a Farr 1/2 ton , any more than 3 in light winds costs time, but in stronger winds, 2-3 more bodies to put weigh on the rail and increases speed.

I guess a "typical cruiser" probably fits somewhere in between them

IOW, it depends on the boat and the weather - but if you're racing, emptying your water tank(s) and leaving your anchor gear behind will probably make more difference since that is unmoving ballast which is probably in the wrong place for performance to start with
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Old 30-01-2016, 16:38   #9
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Re: How much does crew weight affect cruising speed.

Our boat carries 800 litres of water when the tanks are full. It is noticeably faster when the tanks are lower, so that we generally aim for our usage (11l/day) x # of days expected, plus 50%. But then, (a) we're experienced water conservers, and these passages are repeats of others, so that we can estimate with pretty good accuracy. You do need extra, but we do not need to completely fill ours.

If you're the kind of sailor who thinks it's fun to drive the boat, that increase in responsiveness which is a pleasure to you is a big plus, but it still does not greatly affect our average days' runs.

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Old 30-01-2016, 17:00   #10
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Re: How much does crew weight affect cruising speed.

It's not just the weight but the supplies required for that person on a long trip. How much beer does the average yank need, 24oz steaks for a Canadian, or tea bags for a pom?

Also worth thinking about were that weight is located. On a 50ft boat one person isn't going to make much difference, but on our 31ft moving from side to side alters the trim as does everyone sitting in the cockpit. Having removed the genny, moved the life raft to the cabin, 90m of power cable and an anchor to the skip we have raised the stern at rest and probably underway, just a bit more difficult to measure. Does it now go faster? must do surely :-)
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Old 31-01-2016, 09:25   #11
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Re: How much does crew weight affect cruising speed.

Its the old law of inverse relationships: heavier the load, slower the boat. Basic mechanical engineering 101. But is the relationship of practical importance? No, for most boats, their load capacity limit will not materially affect their speed. Overload and it will affect speed. A planing boat will not get up and plane if overloaded(or a lot of tubbies aboard).
However, when it comes to females the lighter the faster. Unless your hunting whales, in that regard go light.
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Old 31-01-2016, 09:49   #12
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Re: How much does crew weight affect cruising speed.

Simple question deserves a simple answer. Extra person has negligible impact on speed. Do you notice differences after filling fuel or water tanks? I doubt it. Guestimate the total weight of your fully loaded and crewed boat. Then calculate what percent increase another 175 # would add. That should convince you.
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Old 31-01-2016, 11:24   #13
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Re: How much does crew weight affect cruising speed.

Well some of us probably count as 2 people.

But, my weight is 1.3% of my boats listed weight. So I'll say I slow the boat down 1.3%.
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Old 31-01-2016, 11:30   #14
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Re: How much does crew weight affect cruising speed.

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Well some of us probably count as 2 people.

But, my weight is 1.3% of my boats listed weight. So I'll say I slow the boat down 1.3%.
My dog weights in at a bit over 200 pounds and does make a difference in getting the dinghy up to speed. Also affect the gas mileage in the honda.
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Old 31-01-2016, 11:36   #15
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Re: How much does crew weight affect cruising speed.

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My dog weights in at a bit over 200 pounds and does make a difference in getting the dinghy up to speed. Also affect the gas mileage in the honda.
I can play this game

When it is just me in my dinghy my 6 hp overboard can give a pretty wild ride. But if I add my wife it really slows it down. Therefore my wife weights too much

But in truth this means little. Since my boat can sail as fast as it can motor I can make a case that my sails equal the power of the 54 hp inboard in good wind conditions. I don't think an extra body means much to that.

And there there's the rail meat argument
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