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Taroona
08-05-2010, 03:41 PM
Wondering if any one can help with this question.

I have a new boat (Yacht), it's displacemrnt is 1542KG 26ft with an auxilary outboard, I think about year 2000.

What sort of fuel economy would I expect to get out it.

hivalley
08-05-2010, 04:16 PM
Hi Taroona,
Miles per gallon are fairly useless on water when you have to factor in wind waves and currents etc. Best to measure fuel efficiency in pounds of fuel used per horsepower developed per hour, this is called "brake-specific fuel consumption” I always work on 0.6 to 0.8 pounds of fuel per hour per horsepower for a carb 2 strokes. Fuel is about 6 lbs per gallon. So 0.8lbs x 8hp = 6.4lbs = 1.06 Gal = 4.01 liters per hour at WOT. Feel free to correct the maths it’s not my best subject then again not real good at English either:tongue:
Cheers

PS. BTW that is US Gallons.

gr hilly
08-05-2010, 04:22 PM
looks pretty close to me.

finga
08-05-2010, 05:50 PM
Sad bit is in that hour chewing up the 4.01l your not going to go too far if the sea is anything but a milk pond.
A lot of boat to move with a 8hp motor.

patrol50
08-05-2010, 06:23 PM
gday mate i have a yamaha 8 hp that i occassionally use as an auxillary for my hartley 18 ft trailer sailer - two probs one is that it is not long enough its a long shaft and any sort of a nasty chop it porposes out of the water - you really need an extra long shaft and i have one of those also a johnson 8 hp extra long shaft that stays in the water that i mostly use on the hartley - and on you boat with that displacement aa 8 hp 2 stk it really is only suitbale for in and out of moorings - you really will need at least a 9.9 hp or a 15 hp better and a 4 stoke better again - and my advice is get the extra long shaft if you can - bit hard to find these days though - cheers rob

cormorant
08-05-2010, 08:40 PM
Speed is related to waterline length of hull and resistance of keel.

from Wiki

Hull speed, sometimes referred to as displacement speed, is a rule of thumb (http://en.wikipedia.org/wiki/Rule_of_thumb) used to provide an approximate maximum efficient speed for a hull. It is only ever an approximation and only applies where the hull is a fairly traditional displacement design. It is usually described as a speed corresponding to a speed-length ratio of between 1.34 and 1.51 depending on which of the limited sources one refers to.
In English units, this may be expressed as:
http://upload.wikimedia.org/math/5/e/2/5e2ac888b0d427578b801bb45ec8efe4.png
where:
"LWL" is the length of the waterline in feet, and"v" is the speed of the vessel in knots The constant may be given as 1.34 to 1.51 knot·ft−½, or in metric units, 4.50 to 5.07 km·h–1·m-½ (where LWL is measured in metres and v is the speed in km/h)
################################33


8hp will push that hull to hull speed in many small seas but windage tide and beam of boat is another whole issue. It must have a low pitch large blade swept area prop ( sail prop - barge prop) on it and motor must reach correct WOT revs otherwise the motor will labour and you will kill it. .

That is a pretty light hull for that length - is it a sports sailing boat?

Yes there will be prop slippage and possibly issues keeping the prop in the water depending on how it is mounted

Just find out fuel consumption of the motor at 4000rpm and prop it accoringly and as others have stated it is dangerous to be in the wrong conditions underpowered.


We regularly use 6hp on 1700kg 24 foot keel yachts and run 6 knots ( hull speed when they have some growth) and do 4.5 hours on 12 liters ( must check exact tank size) but not at full throttle as once hull speed is reached you are just burning fuel to gain ever decreasing incremental gains. With a 15hp we get better effeciency ( swings bigger and more easily avaliable props) until we try and run harder than hull speed and even that tops out at 8 and a bit knots but will push at hull speed in a larger variety of ea and weather conditions

Taroona
08-05-2010, 09:34 PM
Thanks for your replys

It's an Adams 8 built in 1979 light displacement boat with hull speed of 6.67kts and Gerr's formula of 9.87kts. The boats being surveyed on Wed so will find out if it is a long or extra long shaft



http://www.yachthub.com.au/ad_img/5/1/6/1/6/5_2.jpg

Taroona

cormorant
08-05-2010, 11:21 PM
You can use those funny cloth things instead of the motor if you get bored!!! Just joking A 8hp will push it around , watch out for yacht outboards as they get stored on their side low in the bilge area and often get only short runs ( don't get warmed up) rarely flushed so have salt and scale issues in the block more than usual. often mounted too low on the transom in a following sea they can get a fair drenching Stored next to water they just corrode and wear out quicker unless looked after. Constantly run on old fuel etc etc. 2000 model 8 hp long 2nd hand would be worth as little as $400 as a buyer and $600 as a seller on a good day. Less if you know it has been on a yacht and not propped with a sail prop.


Popular boat there will be other owners or a adams owner site and they all know each others boats and will give you a realistic idea of performance under sail and motor , maybe a bit of history on the boat you are looking at.

Taroona
09-05-2010, 08:52 AM
Hey cormorant, I will have the best of both worlds a stink boat (see avatar) and a yacht

cormorant
09-05-2010, 07:45 PM
So do I and all the bloody antifouling, rigging checks, new sails and maintainance as well. Lot of fun though!!!

Don't overpay as yachts are very hard to sell at the minute - not much money about. Drive a hard bargain and get someone else to maintain it. Love the idea of hardstanding so much easier

Taroona
20-05-2010, 09:10 PM
Can anybody tell me how I find out what Model year the 8hp Yammie is.

cormorant
20-05-2010, 10:51 PM
Starboard side transom bracket should have a lablel on it. Serial number , model and production month / year.

Table with year codes on it

http://www.boats.net/parts/search/Yamaha/Outboard/parts.html


Yamaha website has full parts breakdown diagrams