What outboard size for this hull

[Dennis4b]

Hi,<br /><br />After the great replies in the I/O vs O/B thread about a week ago I've been looking at engines a bit more, now is a good time to buy them (end of season and all that).<br /><br />Most of the times when I mention the size of the boat (length 6.2m, 21') and the engine size (85hp) the first reaction is "underpowered". Now the engine is from '79 so HP rated at the crankshaft, but it cruises nicely at 24knots. <br /><br />I'd already been looking at bigbigbig engines, thinking lots of speed and all that, and then today I talked for a while with a friend who knows MUCH more about these things than I do <br /><br />The hull of the boat is a very flat V. This makes the boat very easy to put on plane, and apparently is designed to cruise very economically at 20-30 knots. Trying to push it much over 30 knots though is like trying to get a sailboat on plane. You can hang twin 225 outboards on it (if it holds!) and it will create a big whirlpool, but not much (relative) speed increase. <br /><br />A modern 90hp gives the boat 30 knots. A bit more HP is of course fine for those bad weather and heavy load conditions, but anything more is a waste of money and fuel.<br /><br />The recommendation was a 90hp Honda 4 stroke (as they are apparently very common here and should be available at a nice price), or then the 135HP orbital Merc/Mariner 2 stroke (135 only because orbitals don't come any smaller).<br /><br />Note that here fuel is very expensive, boating is done in island-rock-ridden coastal area's, with "offshore" meaning you are crossing 60nm to the capital of Estonia for some very cheap beer It's not like you trailer your boat to a lake for a day and burn 40 gallons going back and forth at 70mph <br /><br />I was surprised about the hull story but it does make sense. Anyone with some comments on it, or regarding engine choice?<br /><br />Thanks <br />Dennis<br /><br />PS I can post a picture of the hull, but they should also be in the I/O-O/B thread (Practical no, possible? something-or-other)

 

[Bear]

Re: What outboard size for this hull

Like this one but in swedish<br /><br />Hello Dennis!<br /><br />Do you read swedish? Check the link and search for Flipper 620. You'll get over a hundred posts to read.<br /><br />An Evinrude 140 is 2,2 L/nm. Quite a lot. A Yamaha F100 about 1 L/nm @ 21 knots and top speed of 30 knots. Some difference.<br />Why not a 115 hp from 1989-1994?<br /><br />Try the link. There is a lot of Flipper freaks there.<br /><br />/Bjorn (=bear, and yes, very creative)

 

[evin300]

Re: What outboard size for this hull

One question about the currnet 85 hp @ 20 knots. How many rpm is she turning? If you are bogging the engine down to achieve current performance, then your numbers will not be accurate.<br /><br />I dont think the shape of the V has a large effect, the overall size of the hull and wieght of the boat, and wieght distribution affects the ability to plane to a much higher degree. Good Luck

 

[Dennis4b]

Re: What outboard size for this hull

Evin3000, with the math we did, if the engine would run at 5000rpm with the current prop the theoretical max speed would be 30 knots, so if you add slip and the fact we most likely do not turn 5000 rpm 24 sounds reasonable? Maybe drop an inch.<br /><br />Bjorn, a 115hp 89-94 would of course perform very nicely. Currently we get over 2l/nm, which means we wouldn't even make it to Talinn on a full tank! Both Honda, Yahama F100 and Merc Orbital should give 1l/nm or less. This is the main reason we're looking into those engines, the fuel consumption. I was just wondering what kind of HP would be sufficient.<br /><br />As for reading Swedish, a little Thanks for the link!

 

[mnmfors]

Re: What outboard size for this hull

Hello Dennis,<br />This is more of a question than a suggestion, doesnt your boat have a plate on it with max hp and weight?<br />Wouldnt this plate be right on the money? Then play with the prop pitch?<br />I would guess it be best to use the largest motor allowed/recomended, get a prop for your cruising speed. Therefore, turning less rpm's, for maximum engine life. I would guess it is better to sacrifice fuel consumption to trade for engine life.

 

[Dennis4b]

Re: What outboard size for this hull

Mnmfors, yup I've been looking for that plate, but I guess it's a US thing? Never really seen one here, let alone on my boat <br /><br />I've seen them rigged up to 150HP though, with 90 recommended (promises 30knts).<br /><br />Got an offer today to buy a brand new never used Yamaha F100 4-stroke still in the box, very hefty price (ouch!) but a lot below normal retail. Should be worth close to the same after 2 seasons. And I have to decide tomorrow

 

[evin300]

Re: What outboard size for this hull

Just be sure that you dont have to use WOT to achieve a decent cruising speed, your best gas mileage will be somewhat lower than WOT, even gas efficient motors can be hogs at WOT.<br /><br />If your 85hp cant achieve the recomended max rpm range at WOT, you are lugging the motor which is very hard on it, after propping for the correct rpm, will you still be happy with the cruising speed? I say this because you seem to be using this motor as a benchmark for choosing another, which is fine, but this motor has to be set up properly first. A tachometer is really a must IMHO.