markpierce
Master and Commander
- Joined
- Sep 25, 2010
- Messages
- 12,557
- Location
- USA
- Vessel Name
- Carquinez Coot
- Vessel Make
- penultimate Seahorse Marine Coot hull #6
Gasoline isn't allowed on my boat. Thank you.
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.
Gasoline isn't allowed on my boat. Thank you.
...
Have you no wish/need dink w/ gasoline fired ob motor?
What outboard?
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.)
But on gasoline Mark's got his head in the sand.
Art ... mark has a TRINKA
You can't beat a trink.
But on gasoline Mark's got his head in the sand.
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.
[FONT="]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. I’ll have to do more research. Its more than 2 deg, but how much more I don’t know. [/FONT]
[FONT="]The xls template refers to 2 deg or less where SLR >2.5. That’s 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.[/FONT]
[FONT="]My QBBL is 2.5 deg. I don’t 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. [/FONT]
[FONT="]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 boat’s info, SLRmax (maximum possible speed for displacement mode, which calc incorporates DLR) is 1.428 and thus Vmax is 9.8 kn. So I’m 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. [/FONT]
[FONT="]Now to the point of all the above discussion – I don’t see any sharp breaks from SLR 1.1 to 1.5, it’s 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 5’ extension that many folks have done with their Mk 1’s to get a longer LWL. Unsubstantiated reports of an extra 1 kn. I’d like that. I just didn’t like the quotes to do it. I’m also starting to think that the definitive QBBL number Manyboats (and I) would like to see is going to be quite low.[/FONT]
Ok, some more data:
Ocean Alexander 50 Mk1
60,000 # half load
Twin John Deere 6068, 201 HP each
7 kn, 2.7 NMPG
But it seems I have a lead foot. Looking at daily logs for 2000 nm since my repower I typically am getting 1.3 NMPG at 9.3 kn.
I did a bit more research, and here are a couple of very good pdf's. If you used boat design.net then you might remember Eric Sponberg's series on Design Ratios. On his website he compiled the posts into an updated pdf, and I've attached it. Eric's "primer" is very readable, and whilst it is more sail oriented there are great sections for powerboats.
That file lead me to Dave Gerr's article in Masthead. Also available to download, here it is. The key article is p12-17. In it he discusses things raised earlier in the thread. It is also a very readable discussion for folks like us. He has a great graph for how the 1.34 number varies, discussion of the Quarter Beam Buttock line (and the graphic Eric was looking for) and importantly his updated formula for SLR. It is revised from the one in his Propeller Handbook. It is truly worth printing and keeping pages 12-17.....
... trying to "guess" performance from a few tidbits of internet data is ridiculous.
I personally like dollars/nautical mile.....last year it was about $1/NM ...this year should be the same....
Makes trip planning easy....3000 NM trip is gonna cost me $3000 in fuel..
Dear Ann Landers
In May we will be driving from Arizona to Seward Alaska. Part of the driving trip will be on a ferry. How do I calculate MPG for the Suburban? Then during June through September we will be cruising on the West Coast in our boat. I'm really befuddled as to how to calculate efficiency, GPH and NMPG for the entire summer's journeys.
BTW, my credit card which I will use to cover the cost of the ferry, truck gas and boat fuel may have been hacked at Target. Do the bank fees to make good on this count towards my dollars per nautical mile?
Awaiting your reply, I remain,
Anxious in Arizona