outboards

Outboards or inboards, electric, fuel or diesel, cavitaion and torque talk
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northerncat
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Post by northerncat » Mon Aug 07, 2006 6:37 am

just wondering whether anyone has any experience wiht the new 8 hp yamaha hight thrust outboards, these are obvoiusly going to be a smdigen lighter come with power trim and tilt and be a lttle more economical on fuel, the power trim and tilt would be pretty cool

sean

44c
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Post by 44c » Sun Sep 10, 2006 3:45 am

The Yamaha 8 HT looks ok, but it doesnt come in a long shaft version .



I was originally going to get Yamaha 25HT's but at this stage I am leaning toward Honda BF20's. A quick comparison:



BF20 weighs 52 kg, transom height 703mm

Yam 25HT weighs 90kg, trans ht 642mm

Yam 9.9HT weighs 51kg, trans ht 635mm

Yam 8HT weighs 49kg trans ht 557mm.



The Honda has the best power/weight and the highest ransom height. It also comes with power tilt. There is also a power thrust version, which has a low pitch prop designed to work ok in reverse too.



Only real down side is Honda's prices!

northerncat
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Post by northerncat » Sun Sep 10, 2006 5:03 am

40 horses for the same weight as 20 that sounds great i might have to look into this, i could hit 20 knots just with the motors and forget about the rig and sails

sean

puremajek
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Post by puremajek » Sun Sep 10, 2006 6:20 am

44c



Love your work and what a great way to compare. It has made me look again too.



Auto tilt...what a winner. A few builders together ordering 20 units will change the price. This would even interest international Honda sellers. The Yamaha market has a closed shop on discounting as I have been told, however Honda does not and is trying hard to break into this market.



One concern is their speed in backup - spares, repairs, maintenance etc
James
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northerncat
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Post by northerncat » Sun Sep 10, 2006 6:33 am

is the 52 kilo model the base model or the one with power tilt and trim, i just had a look at their models and they seem to have a few in the lineup, obviously youve already done the research for me :-) thanks and could you post your findings for the rest of us arm chair builders

sean

northerncat
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Post by northerncat » Sun Sep 10, 2006 6:36 am

these honda motors have only 25cc more than yamahas 9.9 but they manage to get another 10 hp out of it, quite an achievement i think

sean

44c
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Post by 44c » Mon Sep 11, 2006 8:04 pm

northerncat wrote:these honda motors have only 25cc more than yamahas 9.9 but they manage to get another 10 hp out of it, quite an achievement i think
sean


Yeah the 9.9 is the same capacity as the 15, so probably just derated a bit.



The weight I quoted is from the Honda brochure, for the BF20D3

XRTE which is the extra long shaft, electric start, remote control, power tilt version, so it should be right, but maybe further investigation is in order. No hurry for me just yet, I dont intend to buy big ticket stuff like that until I need to. (Unless a real bargain shows up.)

puremajek
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Post by puremajek » Fri Sep 15, 2006 6:48 am

Well, I have tried to suss out a little on this topic and here’s my next bit.



I am not sure about the Oram or other yachts however, the Easy’s cruise speeds are based on formula using parameters such as waterline etc. With this formula, the designer then comes with an optimum cruise speed for a given weight (this given weight is normally the lightest and needs to be kept in mind). For the

• Easy 32/9.9, this is around the 7.4 – 7.6 kts,

• Easy 37/10.5ss is around 8.3 – 8.6 kts, and

• Easy 11.6 is around 9.3 – 9.7 kts.



Any speed in excess of this is fantastic, especially if under sail. However, with the outboards a balance between optimum speed (given the above figures) verse outboard thrust is required. Most outboards can claim a higher output, higher horsepower, better fuel efficiency bla, bla, bla. But few will supply actual torque figures for ‘high thrust’ output. Even within the Yamaha range of 9.9’s motors, the wrong buy (as has occurred with a current Easy owner) caused a huge headache.



The actual event saw three Easy’s head out for a days (supposed) sailing, with nil wind they motored out. All went well until they rounded a point, where only two got around. The third had to wait for the current to change, before being able to negotiate the turn and join the others. This boat was smaller the other two and they all had 9.9 Yamahas. Its just that the units on the lagging yacht were not ‘high thrust’. I suspect the message here is that one needs to choose carefully and if price dictates the purchase, the alarm bells should be ringing.



The Yamaha 9.9 long shaft high thrust (which I personally do not like) have been built specifically for the ‘House Boat’ market, with high thrust capable of pushing a lot of dead (drag induced) weight. For this reason, they are said to be designed with continual immersion in mind (i.e. they are normally left in the water for very long periods – corrosion being the killer here). I am told that other manufacturers have tried to target this market, with little success and the reasons are:

• Corrosion with continual immersion,

• Higher fuel consumption,

• Thrust vs higher Top Speed ratio, and

• Reputation for reliability.



Having said all this, I believe that the Yamaha 9.9 long shaft high thrust are still the ones to beat (for my particular yacht anyway given other options currently on the market). Yes, I could get larger models to get the extra few knots on my SAIL boat, but this comes at a price, initially on purchase and then the running costs.



They say these units are reliable, very fuel efficient and have one of the better corrosion systems around. The only thing I don’t like about them is their back-up service (here in Australia is said to be woeful). I have had experience with a 15hp Honda and Evinrude and the Evinrude left the Honda for dead on corrosion resistance and maintenance. Hope this assist others in some small way.
James
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44c
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Post by 44c » Sat Sep 16, 2006 9:53 pm

The main difference between a 9.9 and a 9.9 high thrust is the gearcase and ratios. The HT has a bigger reduction, and swings a bigger diameter prop.



This would possibly account for the performance difference you were talking about, but to be frank I dont really understand how if 3 boats were performing similarly under motor, a change in the current would make that much difference,(given that the boats are moving through the same water, regardless of whether or not the water is moving) unless the water became much more choppy, in which case the size and depth of immersion of the propellor would become important.



Given that power is basically the product of torque and revs, and the fact that both motors have similar rev ranges, it's fairly safe to say that a 20 hp motor will produce more torque than a 10 over most of the rev range. (and at peak hp, the 20 will be producing twice as much)



In my case I would prefer bigger motors than 9.9hp, (although there is an Oram 44c successfully sailing with a pair of Yamaha 9.9HT's currently) and the fact that the Yam25HT weighs nearly twice as much as the Honda BF20, and also that the BF20 has a longer shaft and can also swing a 10.5 inch prop in "power thrust" form is why I am tending to lean in that direction. ( For a total 10 hp less I save nearly 80kg) I have a friend in the NSW waterways dept, and he tells me that Honda's corrosion treatment has improved considerably in recent years too.

northerncat
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Post by northerncat » Sun Sep 17, 2006 1:21 am

i was going to say that the 20 should be able to swing the same size prop if not bigger due to the increased size of the motors, i think 44c you should work faster and put your boat in the water with the 20's so we can have a performance review.

do the hondas have a high(power) thrust option ? and if so what is the gear ratio as i couldn't find any information on this

sean

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