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Old 12-27-2013, 08:59 PM   #61
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Gasoline isn't allowed on my boat. Thank you.
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Old 12-27-2013, 10:21 PM   #62
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I'll be interested in Fred's views as well. His rules of thumb are pretty good.

For what its worth, I have found the Hawaii Marine Company spreadsheet for Displacement and SemiDisplacement hulls pretty good also.

Displacement and Semi-Displacement (or Semi-Planing) Hull Powering Calculations, Description

It uses material from Gerr's Propeller Handbook amongst other sources. It has a small fee and user licence so I cant post the xls file I played around with using some of Art's info. I had to guess a few things, but for Art's Tolly I got a figure for SLR of 1.37 from it, giving 'hull speed' of 7.7 kn.

The xls file seems to manage the FD versus SD hull type issue reasonably well. For SD hulls, like my own, I find that SLR of 1.2 is still pretty economical. For Art, 1.2 is a speed of 6.8 kn so it seems to fit for his Tolly as well.
TY, Brian
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Old 12-27-2013, 10:26 PM   #63
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Gasoline isn't allowed on my boat. Thank you.
Mark - Don't be Scared! ... Ya Simply Gotta be Careful!!

Have you no wish/need for a dink w/ gasoline fired ob motor?
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Old 12-27-2013, 10:32 PM   #64
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Insequent,
Good post and link. I was especially interested in what he calls "aft quarter butt exit angle". I've refereed to this several times as the QBBL or "quarter beam buttock line". He made mention of explanatory graphics but I found none. I was hoping to see a definitive angle that would define SD and FD hulls.

But there is more to it than just the QBBL angle. There are lots of boats w straight QBB lines and many (or even most) have curved buttock lines. There are also hooked aft buttock lines that are obviously concave usually to keep a SD hull from squatting too much. If they achieve that they usually make a big bow wave and are wet. But some just squat anyway and some even turn out right. But most FD hulls have curved (convex) buttock lines. I offer my own Willard as an example. Very nonlinear as well.
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Old 12-27-2013, 10:47 PM   #65
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...

Have you no wish/need dink w/ gasoline fired ob motor?
What outboard?

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Old 12-27-2013, 11:07 PM   #66
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What outboard?

Mark - I am impressed at how much you look like Popeye! Congrats!!

BTW: I was a competing rower in one of my early lives... Wanna race??

I really enjoy our 50 hp 39 knot WOT tow behind gasser "dink"!
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Old 12-27-2013, 11:28 PM   #67
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Sounds like fun, Art, but your "gasser dink" doesn't appear to be able to mount on the Coot. (Rowing a boat was my first "real" boating experience, which I recall was on Clear Lake, CA some fifty-plus years ago.)
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Old 12-27-2013, 11:34 PM   #68
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Racing? This looks like the way to go!



(Why are they all looking at me and not rowing?)
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Old 12-28-2013, 12:04 AM   #69
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Sounds like fun, Art, but your "gasser dink" doesn't appear to be able to mount on the Coot. (Rowing a boat was my first "real" boating experience, which I recall was on Clear Lake, CA some fifty-plus years ago.)
Mark

On Coot, no need to mount a comfortable "4-seater" cruiser dink that is economical to run, powerful and fast. Short tow line or side tie for close quarters / slow speeds (canals, docking, fuel stops and the like). Longer tow line for cruises at speed. A tow behind "dink" (gunk holen run about) with good hull design will act well in even fairly rough seas. Fender pleasingly bolted to tow behind dink’s bow allows slow speed transom bumps during close-maneuvers leaving no marks. Often towed a 13' 3” Boston Whaler w/ 40 hp Johnson throughout New England coastal and inland waters for many years during 20th Century's 3rd quarter.

We love to tow our 14’ 8” 50 hp Johnson Crestliner “Stinger” behind our Tolly!

Try It - You'll Like It!
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Old 12-28-2013, 12:17 AM   #70
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Art ... mark has a TRINKA

You can't beat a trink.


But on gasoline Mark's got his head in the sand.
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Old 12-28-2013, 12:43 AM   #71
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But on gasoline Mark's got his head in the sand.
I wouldn't say that Eric. Mark broadened his horizons recently and moored 4' away from a gasser. Bliss felt privileged to accompany the Coot for an evening.
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Old 12-28-2013, 12:47 AM   #72
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Art ... mark has a TRINKA

You can't beat a trink.


But on gasoline Mark's got his head in the sand.
Even ample gasoline fumes emanating from sand can explode to flame... Mark you've no place to hide! Fear can kill!! - LOL!

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Old 12-28-2013, 02:18 AM   #73
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Insequent,
.....I was especially interested in what he calls "aft quarter butt exit angle". I've refereed to this several times as the QBBL or "quarter beam buttock line". He made mention of explanatory graphics but I found none. I was hoping to see a definitive angle that would define SD and FD hulls.
Eric, if you bought the Hawaii Marine xls template the graphic is on the 'References' tab. And that is taken direct from Dave Gerr's Propeller Handbook, p13. My copy of the book is on the boat so I cant check Gerr's discussion of it at present. You now have me curious about the QBBL for D to SD transition. Iíll have to do more research. Its more than 2 deg, but how much more I donít know.


The xls template refers to 2 deg or less where SLR >2.5. Thatís getting toward the SD to planing transition. At 17 kn Art's Tolly gets to SLR 3, maximum possible for SD mode (Gerr). So his installed power is a good balance with his hull shape.

My QBBL is 2.5 deg. I donít believe I get near proper planning. I think this is in part due to QBBL and in part too much weight. When delivered new, boats like mine all had trim tabs fitted to try and counteract stern squat and keep the bow down. With trim tabs, the boat could get to 17kn at WOT. This is an SLR of about 2.48. It seemed to me that using trim tabs for this purpose was just forcing a hull shape to perform above where it should, at a cost of high HP and fuel requirements.

Above about 10.5 kn I really noticed the stern squat and got a feeling of plouging through the water rather than planing. It was still noticeable at 16 kn. So I normally would not try for more than 10 kn SOG unless I had current assistance. Considering this, when I repowered I went down to 402 HP from 540 HP. I now get 11 kn at WOT, without trim tabs (I removed them) for an SLR of 1.6. But 10 kn SOG without current assist still has a reasonable bow attitude. This is my high speed cruise mode. Using the Gerr formulae and my boatís info, SLRmax (maximum possible speed for displacement mode, which calc incorporates DLR) is 1.428 and thus Vmax is 9.8 kn. So Iím into the SD mode, but not into it very far before my QBBL and weight give serious stern squat and really hurt performance. If I go back to 7.5 kn, SLR is about 1.1. That is clearly displacement mode, and my low speed cruise or economy setting.

Now to the point of all the above discussion Ė I donít see any sharp breaks from SLR 1.1 to 1.5, itís a smooth change in HP or fuel used and speed change. My preliminary conclusion is that for me to get good SD performance, QBBL should be less or the boat should be returned to its as-delivered light ship weight. Neither is feasible. But what I can do is add the 5í extension that many folks have done with their Mk 1ís to get a longer LWL. Unsubstantiated reports of an extra 1 kn. Iíd like that. I just didnít like the quotes to do it. Iím also starting to think that the definitive QBBL number Manyboats (and I) would like to see is going to be quite low.
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Old 12-28-2013, 05:37 AM   #74
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The most efficient boat, don't burn any fuel
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Old 12-28-2013, 07:05 AM   #75
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Eric, if you bought the Hawaii Marine xls template the graphic is on the 'References' tab. And that is taken direct from Dave Gerr's Propeller Handbook, p13. My copy of the book is on the boat so I cant check Gerr's discussion of it at present. You now have me curious about the QBBL for D to SD transition. Iíll have to do more research. Its more than 2 deg, but how much more I donít know.

The xls template refers to 2 deg or less where SLR >2.5. Thatís getting toward the SD to planing transition. At 17 kn Art's Tolly gets to SLR 3, maximum possible for SD mode (Gerr). So his installed power is a good balance with his hull shape.

My QBBL is 2.5 deg. I donít believe I get near proper planning. I think this is in part due to QBBL and in part too much weight. When delivered new, boats like mine all had trim tabs fitted to try and counteract stern squat and keep the bow down. With trim tabs, the boat could get to 17kn at WOT. This is an SLR of about 2.48. It seemed to me that using trim tabs for this purpose was just forcing a hull shape to perform above where it should, at a cost of high HP and fuel requirements.

Above about 10.5 kn I really noticed the stern squat and got a feeling of plouging through the water rather than planing. It was still noticeable at 16 kn. So I normally would not try for more than 10 kn SOG unless I had current assistance. Considering this, when I repowered I went down to 402 HP from 540 HP. I now get 11 kn at WOT, without trim tabs (I removed them) for an SLR of 1.6. But 10 kn SOG without current assist still has a reasonable bow attitude. This is my high speed cruise mode. Using the Gerr formulae and my boatís info, SLRmax (maximum possible speed for displacement mode, which calc incorporates DLR) is 1.428 and thus Vmax is 9.8 kn. So Iím into the SD mode, but not into it very far before my QBBL and weight give serious stern squat and really hurt performance. If I go back to 7.5 kn, SLR is about 1.1. That is clearly displacement mode, and my low speed cruise or economy setting.

Now to the point of all the above discussion Ė I donít see any sharp breaks from SLR 1.1 to 1.5, itís a smooth change in HP or fuel used and speed change. My preliminary conclusion is that for me to get good SD performance, QBBL should be less or the boat should be returned to its as-delivered light ship weight. Neither is feasible. But what I can do is add the 5í extension that many folks have done with their Mk 1ís to get a longer LWL. Unsubstantiated reports of an extra 1 kn. Iíd like that. I just didnít like the quotes to do it. Iím also starting to think that the definitive QBBL number Manyboats (and I) would like to see is going to be quite low.
Brian... yours, Fredís, Ericís and some other TF members inputs are priceless.

Add to my boatís stats - post 48: Tollyís top condition 255 hp gassers push her 16 to 17 knots (at 1 +/- nmpg); just before quadrajet secondary opens (3,300 +/- rpm with trim tabs in play). WOTís 4,400 rpm reaches 21/22 knots (.05 nmpg Ė if lucky, Ouch!). We often cruise with twins pushing us at 6.5 to 7 knots for about 2 nmpg (3 + gph). For lackadaisical loafing-along we sometimes run on either one screw at 5 + knots and around 2.75 nmpg. If weíve need to cover distance rather quickly the 1 nmpg 16/17 knot full plane speed is enjoyable, and rather quiet too (especially on fly bridge) w/ synchronized gassers in well insulated engine compartment under salon sole.

Believing in 20% safety-fuel reserve (40 gals in our case): Tollyís 2 Ė 100 gal tanks give us approx 160 mile range using twin screws at 16 + knot full plane / 320 miles at 6.5 knots / 440 miles at 5 knots on one screw.

Not too bad for a beefy, comfortable, sea worthy, and fully self contained pleasure cruiser (sometimes referred to by others as a ďTrawlerĒ Ė lol)... We do like our easy to live with Tollycraft. Sheís affordable and reliable as all get out!

Nmpg, gph and mile range calcs are considering our Tolly fully loaded... but, they do not take into account tow behind runabout or gen set usage. I deduct 10% off these calcs regarding on-board fuel-stock as compared to range availability when both those added factors are included for a cruise.
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Old 12-28-2013, 08:35 AM   #76
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Hereís the trip summary for both Colombia to Aruba. We had some positive current and 15 kts on the nose (average).

279 miles 1.46 gph 3.82 mpg

And hereís the trip summary from Bonaire to Trinidad: 425 miles. We had contrary current of at least 1 knot and as much as 2.5 knots for the entire trip.

First 224 miles: 1.5 gph 3.2 mpg
Last 201 miles: 2.0 gph 2.0 mpg

The first half we had lighter head winds and less current. We typically cruise at 6-6.5 knots, burn 1.8 gph and get 3.8 mpg with light winds and no current. The paravanes were in the water the entire trip which costs us about 3/4 of a knot.
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Old 12-28-2013, 10:04 AM   #77
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Thanks for the info guys, all very usefull numbers. However, to be able to compare apples with apples the base criteria must be the same for all. Speed being the most significant factor here. I picked 7 knots because most 40 foot lwl boats can do that economically. 6 knots is even more economical and is also about the average speed my trawler makes on longer trips. We should not add in generator usage or towing a dink, Stabilization is a factor that is more difficult to account for, but on a round bottom trawler I would say we just have to eat that fuel usage because it is absolutely neccessary. A flatter bottom has more initial stability and doesnt need as much or any stabilization. So, we can call stabilizers just part of the ship for these purposes. If we calculate into the equation the lower cost of gasoline and the lower/lesser maintenance cost of a gas engined boat we may be surprised at the difference. My trawler is 52 foot oa, 42 foot lwl, 16 foot beam, about 50,000 lbs, Krogen designed full displacement hull, with twin 4-53 DDs at 120 hp each. At 7 knots (IIRC) we got about 2.75 mpg. This boat has been in the hard for several years now for a refit. My sportfisher is 50 foot oa, 43 foot lwl, 16 foot beam, about 48,000 lbs. pure planing designed hull, with DD 8v92s at 550 hp each. Top speed is 30 knots, making OPEC smile. At 7 knots on 1 engine I can get 2.5 mpg. At 10 knots it gets about 1.5 mpg. Keep the numbers coming. Thanks
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Old 12-28-2013, 11:50 AM   #78
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Insequent, Kulas, Larry and Art,
Best discussion on speed, hull shape and power we've ever had.
I assume that your (Insequent's) boat is similar to Daddyo's. I've seen his pics and other boats out of the water. I'm still reading and absorbing your post #79. Especially your last paragraph. " Iím also starting to think that the definitive QBBL number Manyboats (and I) would like to see is going to be quite low." To make a SD hull closer to a planing hull a lower. Increasing the angle (in numerical terms as in 6 degrees rather than 4 degrees) would increase efficiency and require the boat to go slower to realize the increased efficiency.

Oh I just figured it out. You're referring to the number of the QBBL angle that would divide FD hulls from SD hulls. I read about this on BoatDesign.net and someone gave a definitive number to this end. I don;t recall the number, it's source or if the number just came out of somebody's head. Could be the latter but I suspect not. I searched on BD.net several times but came up empty handed. However if such a number was found or established it still would only be a basic discussion point that would give some scope to this issue as there are so many other variables. Variables like if the displacement was carried in close to the keel or out toward the chines. An extreme example of the latter would be a barge.

AS I pointed out earlier a relationship between the cross sectional area of the deepest portion of a hull compared to the area of the submerged transom could also be a rule of thumb to the same end and of course have the same limitations if not more. I carried the opinion for some time that any boat w it's transom above the WL identified itl as FD and any "significant" amount of submerged transom identified a SD hull.

To study, evaluate and learn about this is one thing .. of interest to only a limited number here of TF but to have a rule of thumb to quickly identify a SD boat from a FD boat would have very significant value to many. I say this as there seems to be an extremely wide range of opinions as to what a FD and SD hull is and how to identify the each.

Two avenues toward definitive success would be to establish numerical documentation that would put every hull neatly in it's appropriate box or to narrowly define the capabilities of each, measure the performance of each and put in the appropriate box. That could be useable or beneficial to the NA but of little use here on TF.

It's fun talking about this and probably leads to bits of new knowledge for many. That in itself makes it worth discussing even if we fail to establish a definitive rule of thumb.

Since I opened the door to all for coffee at Barnes & Noble in Bellingham WA this morning (10:30) I better get going so as to not be a no-show.
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Old 12-28-2013, 04:51 PM   #79
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My sportfisher, with a fairly sharp bow entry angle, if ran bow down and stern clear of the water would come close to FD. It was not designed to run that way but it is doable and to some extent at low speeds this is exactly how it runs. Add some power and the big props will really drop the stern, like almost 12 inches. a lot more at higher settings. At 25 knots the wake is substantial. My trawler has this design built in, and no flat planing area aft. The wake it leaves is very small.At 7.5 knots the wake behind my sportfisher is about the same. Mpg is not that much different and with modern electronic diesels even at 600 hp the mpg would be almost indecernable between the two. Possibly, the trawler could benifit from the new tech, but that has not been proven. It still takes about the same amount of diesel to push the same size, weight and approximate hull configuration thru the water at 7 knots.
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Old 12-28-2013, 05:09 PM   #80
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As you can tell via my general calcs as well as last paragraph on post # 75 - I believe in and recommend caution regarding calculating fuel usage and therefore the reserve supply. Ya just never know when or why those precious last drops of fuel might be required!
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