Cruiser Displacement and Sail Area - Page 3

Joli · 22-10-2009, 15:29

This is kind of like the previous discussion. Assuming similar hull shapes, moments....

Boat 1, (the heavy) will be quicker when loaded for cruising gear because it will retain more "horsepower" as the gear is added.

Boat 2, (the light) will be quicker when empty because it will have a longer waterline but will carry less "total" sail although the horsepower "SA/D" is the same for both.

Here's the data, making some assumptions:

Heavy ~ 40 loa, 34.7 lwl, d 30,000#, sa/d 17.5, sa 1055, d/l 320

Light ~ 40 loa, 35.5 lwl, d 25,000#, sa/d 17.5, sa 935, d/l 250

Beam for both is 12.

sa/d with cruising gear of5k: Heavy~15.8, Light~15.5

The more gear you carry the more starved for power the light boat will be. The heavy boat will have a bigger rig and you can proportionately increase the sa/d more.

Quote:

Originally Posted by Don Lucas

When looking at 2 different boats of same length; what is the sailing performance difference for "normal" conditions (not real light air like 5 knots) between the boats with different displacement, but same displacement/sail area? Say for the 2 boats below:

Boat 1
displacement - 30,000
DLR - 320
displacement/sail area - 17.5

Boat 2
displacement - 25,000
DLR - 250
displacement/sail area - 17.5

Both have the same power ratio for their displacement so would seem to me to have same sailing performance in general. Given the heavier boat is going to be more comfortable and I think may be faster at higher wind because it may standup better at the higher wind before having to reef.

If you have any real ratios etc from your experience to go along with your answer that would be nice.

Thanks

Hiracer · 22-10-2009, 16:05

I truly believe it's harder to make a good heavy boat, than a good light boat. A heavy design has more limitiations.

Not that it can't be done. It can. But frequently isn't.

If a person doesn't understand design somewhat, I think it best to hire an expert like Bob P, or stick to mid-weight boats.

Joli · 22-10-2009, 17:29

Tell that to Ted Hood?

Quote:

Originally Posted by Hiracer

I truly believe it's harder to make a good heavy boat, than a good light boat. A heavy design has more limitiations.

Not that it can't be done. It can. But frequently isn't.

If a person doesn't understand design somewhat, I think it best to hire an expert like Bob P, or stick to mid-weight boats.

Hiracer · 22-10-2009, 17:50

Quote:

Originally Posted by Joli

Tell that to Ted Hood?

Yep, I like Ted Hood boats. He promoted centerboarders more than a left coast guy like me does, but otherwise I likie.

Hiracer · 23-10-2009, 11:34

I had an IOR inspired boat with fine ends (the old diamond shape hull). She was a fantastic sailor, but in certain short steep seas she would hobbyhorse. A fatter stern would have help dampen that.

I think it's all sea state dependant. Just about any boat can misbehave if the sea state is pefectly 'wrong' for that boat.

* * *

Yes, depth cures lots of problems, but creates another one.

S&S · 23-10-2009, 12:42

Quote:

Originally Posted by Hiracer

I had an IOR inspired boat with fine ends (the old diamond shape hull). She was a fantastic sailor, but in certain short steep seas she would hobbyhorse. A fatter stern would have help dampen that.

I think it's all sea state dependant. Just about any boat can misbehave if the sea state is pefectly 'wrong' for that boat.

* * *

Yes, depth cures lots of problems, but creates another one.

Aah, now I understand- ours is a CCA boat (she's skinny all over but draws 8'). No hobbyhorsing but she's a wet ride to windward.

Hiracer · 23-10-2009, 14:02

It's a crime what IOR did in the evolution of boats from the CCA rule.

Cruising boats should be completely blind to racing rules, but unfortunately that's not always the case. Witness the racer/cruiser designation.

Eight feet draft: I'm jealous.

Pretty boat, btw. In the picture in the slings, draft doesn't look like a full 8 feet but I'll take your word for it. I would have guessed 7 feet.

What's your B/D ratio?

Sweet boat.

S&S · 23-10-2009, 14:06

Quote:

Originally Posted by Hiracer

It's a crime what IOR did in the evolution of boats from the CCA rule.

Cruising boats should be completely blind to racing rules, but unfortunately that's not always the case. Witness the racer/cruiser designation.

I'm in complete agreement.

Quote:

Eight feet draft: I'm jealous.

We do have to use the dink- a lot. there're always compromises.

S&S · 23-10-2009, 15:25

Quote:

Originally Posted by Hiracer

Pretty boat, btw. In the picture in the slings, draft doesn't look like a full 8 feet but I'll take your word for it. I would have guessed 7 feet.

8' from the LWL, probably 7' 10" /7' 11" in real life. Lines drawings say 7'11" but it's 8 if I put a lot of stuff on.

Quote:

What's your B/D ratio?

Not quite sure of the exact weight so calculated to be somewhere between 48 and 52%. Derecktor didn't follow the plans exactly and some mods were done early on. About 18K lbs lead and 34K disp.

Quote:

Sweet boat.

Thank you!

RainDog · 23-10-2009, 14:06

As several others have mentioned, do not put too much stock in the DLR. It can be misleading.

As an example compare the Pacific Seacraft 40 to the Valiant 40. They have nearly identical rigs (the PS has a longer boom), displacement, LOD, beam, draft, and PHRF. They also have similar underbodies. I do not know the wetted surface area, but for the sake of argument, let us assume they are in the same ballpark.

However the Valiant 40 is almost 3 feet longer on the water line. This gives the PS a DLR of 335 while the Valiant is 255. This makes the PS a "Heavy Displacement" cruiser and the Valiant is firmly in the "Moderate Displacement" range. But what does this really mean? Since they have nearly identical rigs and displacements, they will sail similarly in most conditions. In theory, the extra three feet on the water line means the Valiant can sail 7.8 knots vs. 7.5 on the PS. However in cruising conditions, I think few cruisers would often sail these boats at hull speed. If they did that would be 187 mile days for the Valiant vs. 180 mile days for the PS -- still not much of a difference. In light conditions, since they have the same displacement, similar rigs, and similar underbodies, I think they would also have very comparable performance. The PHRF numbers seem to verify this.

I am not saying the DLR is useless, just that it should be treated with care because it can be deceiving.

Hiracer · 23-10-2009, 14:11

Quote:

Originally Posted by RainDog

I am not saying the DLR is useless, just that it should be treated with care because it can be deceiving.

Agree completely. It's one of the least 'dimensionless' numbers.
Also, I think the bigger the boat the easier to go lighter.

Don Lucas · 23-10-2009, 15:08

Not disagreeing with the above, but I don't believe I posted this as DLR question. It was really about displacement and sail area. Yes I gave a DLR for the 2 boats which changed with the displacement as one would expect.

Some of those old heavy Hood boats only had a power ratio of 14.5. Not going to go anywhere fast, but going to comfortable while doing it so guess that is agood thing.

22-10-2009, 16:05	#2
Hiracer Registered User Join Date: Oct 2006 Location: The boat lives at Fidalgo Island, PNW Boat: 36' custom steel Posts: 602	I truly believe it's harder to make a good heavy boat, than a good light boat. A heavy design has more limitiations. Not that it can't be done. It can. But frequently isn't. If a person doesn't understand design somewhat, I think it best to hire an expert like Bob P, or stick to mid-weight boats. __________________ John, sailing a custom 36' double-headed steel sloop--a 2001 derivation of a 1976 Ted Brewer design.

23-10-2009, 11:34	#5
Hiracer Registered User Join Date: Oct 2006 Location: The boat lives at Fidalgo Island, PNW Boat: 36' custom steel Posts: 602	I had an IOR inspired boat with fine ends (the old diamond shape hull). She was a fantastic sailor, but in certain short steep seas she would hobbyhorse. A fatter stern would have help dampen that. I think it's all sea state dependant. Just about any boat can misbehave if the sea state is pefectly 'wrong' for that boat. * * * Yes, depth cures lots of problems, but creates another one. __________________ John, sailing a custom 36' double-headed steel sloop--a 2001 derivation of a 1976 Ted Brewer design.

23-10-2009, 14:02	#7
Hiracer Registered User Join Date: Oct 2006 Location: The boat lives at Fidalgo Island, PNW Boat: 36' custom steel Posts: 602	It's a crime what IOR did in the evolution of boats from the CCA rule. Cruising boats should be completely blind to racing rules, but unfortunately that's not always the case. Witness the racer/cruiser designation. Eight feet draft: I'm jealous. Pretty boat, btw. In the picture in the slings, draft doesn't look like a full 8 feet but I'll take your word for it. I would have guessed 7 feet. What's your B/D ratio? Sweet boat. __________________ John, sailing a custom 36' double-headed steel sloop--a 2001 derivation of a 1976 Ted Brewer design.

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23-10-2009, 14:06	#10
RainDog Registered User Join Date: Jun 2009 Location: Austin, TX Boat: Pacific Seacraft 34 #142 Posts: 67	As several others have mentioned, do not put too much stock in the DLR. It can be misleading. As an example compare the Pacific Seacraft 40 to the Valiant 40. They have nearly identical rigs (the PS has a longer boom), displacement, LOD, beam, draft, and PHRF. They also have similar underbodies. I do not know the wetted surface area, but for the sake of argument, let us assume they are in the same ballpark. However the Valiant 40 is almost 3 feet longer on the water line. This gives the PS a DLR of 335 while the Valiant is 255. This makes the PS a "Heavy Displacement" cruiser and the Valiant is firmly in the "Moderate Displacement" range. But what does this really mean? Since they have nearly identical rigs and displacements, they will sail similarly in most conditions. In theory, the extra three feet on the water line means the Valiant can sail 7.8 knots vs. 7.5 on the PS. However in cruising conditions, I think few cruisers would often sail these boats at hull speed. If they did that would be 187 mile days for the Valiant vs. 180 mile days for the PS -- still not much of a difference. In light conditions, since they have the same displacement, similar rigs, and similar underbodies, I think they would also have very comparable performance. The PHRF numbers seem to verify this. I am not saying the DLR is useless, just that it should be treated with care because it can be deceiving.

23-10-2009, 15:08	#12
Don Lucas Registered User Join Date: Jul 2007 Location: Salem MA Boat: Cal 39 - D & D Posts: 486	Not disagreeing with the above, but I don't believe I posted this as DLR question. It was really about displacement and sail area. Yes I gave a DLR for the 2 boats which changed with the displacement as one would expect. Some of those old heavy Hood boats only had a power ratio of 14.5. Not going to go anywhere fast, but going to comfortable while doing it so guess that is agood thing.