9.9hp motor for BC23

[BC23 Skylark]

My Bay Cruiser 23 has the 6hp Yamaha motor. The tide in our river is fairly strong so I am thinking about replacing it with a Yamaha 9.9hp motor with an electric start. Does anyone have this motor in their boat? Does it protrude into the cockpit? Does the boat require any modification to fit the larger motor? Does the extra weight (12kg) affect the trim significantly?

[Philip L]

When I ordered the larger motor, I asked Matt about trim and he didn't think it would be a problem.  It fits without problem but is heavy to lift (42kg?).  Have attached a photo so you can judge for yourself.  I don't think it is significantly larger than the 6hp and doesn't protrude into the cockpit - once more photo attached.   
The electric start is great but it still pull starts easily.  You will need to consider if your battery has enough cold cranking amps - I think the manual says 150 which seems like a lot to me!  It will mean you get some charge back when motoring.  My battery monitor has recorded 3-4 amps while motoring at about 4 knots.  One other advantage is the reduced noise.

[Philip L]

Trim picture

[Martijn]

The tide in our river is fairly strong

I have the 8HP version, which I think is identical to the 9.9 HP only tuned down to produce less HP.
It fits perfectly without modifications and provides more power than required. It is also very quiet compared to the single cylinder 6HP I used previously.

The 8HP will already drive the boat at hull speed at appr. 2/3 of throttle. Since hull speed is measured relative to the water (including tide!) I would see no benefit to opt for the more expensive 9.9 HP engine. It will certainly not make the boat go faster against the tide than the 8HP. Both versions are available with electric start or can be easily converted using an OEM upgrade kit.

[Sea Simon]

Beware..it's not quite as straight forward as a "hull speed" limitation...IMHO.

Speed will be affected by both resistance and displacement (weight carrying "load") and an increase in either, for example, will require more power for required/given speed.

Eg, I recently had to move for a friend a very heavily fouled small motor boat to its winter berth. Its 9hp diesel inboard would barely  move the boat. Both prop and hull heavily fouled. At 2/3 throttle this boat would make way at all.Theoretical hull speed irrelevant in this case.

The 8 v 9.9 debate would be more about price/affordability  and perhaps availability, imho.
Isn't the 9.9 model due to legal constraints (10hp power limit)  imposed on our continental chums, and also adopted by some insurers?

My 5HP Honda was considerably  cheaper than the 6, but perhaps more importantly it was in stock.
However I also seem to recall that a charging coil was a factory option on the 6, but not the 5hp honda? Therefore a "special order".

[Martijn]

Beware..it's not quite as straight forward as a "hull speed" limitation...IMHO.

For displacement boats of this size, waterline length (and therefore hull speed) is ultimately more a limiting factor than displacement.
Besides, Skylark's problem is a lack of speed against strong tide, and I think we can assume the displacement of the boat with new engine will not differ much from it's current displacement.

Although most people know boats have a (theoretical) maximum hull speed which depends on the waterline length of a boat, not all are aware that this speed is relative to the water.
The maximum hull speed for a BC23 is appr. 6.3 - 6.5 kn. Sailing against a 3 kn. tide, the speed over ground will never be much higher than 4.5 kn. regardless of engine power.
In other words, if Skylark can squeeze 6.5 kn. out of the current 6 HP Yamaha (and I bet that's possible), the 10HP engine will not make it go significantly faster against a tide.

What it WILL do is move the boat at the same 6.5 kn. with less throttle, less noise and less vibrations. It will also help overcome additional wind and/or wave resistance (should you need it). All of which is also done by the 8HP engine.

OR, when moving at full throttle, it will make nice big waves at around 6.6 kn., at the expense of a considerable amount of extra fuel.

Of course there can be (many) other reasons to opt for a specific engine, but Skylarks question indicates he needs more speed against a tide. I was merely pointing out that the 8HP equivalent engine might be an equally sufficient but less expensive alternative, especially since both engines will drive the boat at the same speed of 6.5 kn.

[Martijn]

The electric start is great but it still pull starts easily.  You will need to consider if your battery has enough cold cranking amps - I think the manual says 150 which seems like a lot to me!

150 A is indeed way too high! it's probably the maximum rated power of the starter engine.

My battery monitor shows my 8HP Yamaha (which as mentioned earlier is identical to the 9.9HP) peaks at 25A during a start. All my electrics run from a LiFePo4 battery which will not work with the charger of an outboard. I therefore use a dedicated and very small motorcycle starter battery (only 3.1 kg) for starting the Yamaha. Although the battery is rated to deliver 210 Amps during a start, it can only take a continuous charging current of 6 A. Might be worth noting.

[Sea Simon]

Thank you M, v interesting. I guess you may be a Naval Architect, or similar?

"I think we can assume the displacement of the boat with new engine will not differ much from it's current displacement."

I was really thinking about variations  in crew weights, personal effects etc. singlehanded versus family cruising, for example.
Here the associated changes in displacement result in a relatively  large changes relative to lightship weight. (You could easily load 500kg, imo?). To perhaps risk a "dance on the head of a pin", this will affect waterline length and resistance, to some degree.

Of course, the engine forms part of the light ship weight, strictly  speaking.

OP.
Simply put: fully loaded, against wind and/or tide, you'll be pleased you have more power in reserve (for several reasons, some of which M has highlighted)...question is, imho, is the additional cost "reasonable", given (as M rightly points out) associated increase in engine weight is in itself negligible.

I am a Marine Engineer NOT a Naval Architect  therefore not expert!
...and then there are medium term power output & other "losses" due to aging of hull and machinery...that's  more my area, unfortunately!


At the time, my Honda  5 was about 35% cheaper than a 6 and with immediate availability not a 3 month wait. I had no engine at the time...
Honda UK site today shows an 8 at £2445, and a 10 at £2515.
A 10 for me then please! However, both are out of stock.
https://store.honda.co.uk/marine.list
Both with v similar spec, and both at 42kg.

[Martijn]

Hi Simon,

I'm a NA, although professionally more specialized in intact/damage stability of large ships than design.

I was really thinking about variations  in crew weights, personal effects etc

I figured that. According to the CE tag the BC23 has a maximum load of 600 kg. This includes all weights except for the bare boat, mast and sails (so crew, engine, fuel, fresh water etc). This means that in practice, without changing water ballast, displacement is likely to vary no more than 350 kg, for which the change of length on waterline is negligible (maybe 2 - 3 inch). Of course the additional weight requires additional power to push forward, but also not that much.

Of course, the engine forms part of the light ship weight, strictly  speaking.

Not for boats fitted with outboards, as these can be removed (or replaced with larger ones  ), which is why the outboard weight is included in the CE maximum load. If you would add a 100 HP outboard, the heavier engine would reduce the crew carrying capacity to 4 rather than 5-6.

You would have plenty of power to spare though

[Philip L]

Back at home now.
Actual Battery requirements in the F9.9J manual are:-
Battery Rating (CCA/EN): 347-411A
Battery Rating (20hr/IEC): 40Ah

I understand that the 20hr rating means that the battery will deliver 2 amps for 20 hours before being fully discharged.  Does anyone understand the CCA rating of 346-411?

[BC23 Skylark]

Thank you for your replies. I would like the motor to be less noisy too. I am encouraged to go for the larger motor, but am no electrician, so might think twice about the electric start. I suppose a 2 cylinder motor starts more easily than a single cylinder one.

[Sea Simon]

No electrician either, see above....but imho the e starting is probably the easy bit.

The difficulties seem to come with interfacing/connecting plotters/sounders/ autopilots etc etc with what is essentially  a very crude magneto type outboard engine "charge system"....in a small wet boat...

[Sea Simon]

And as to easier starting...IMHO..."probably"...

Over the years, I've had several hand start, tiller steered 40HPs (I think the latest one was a 3 cyl?) and thought nothing of it...but I was younger then, and even had a car with manual windows and no AC 

[Sea Simon]

quote]Of course, the engine forms part of the light ship weight, strictly  speaking.[/quote]
Not for boats fitted with outboards, as these can be removed (or replaced with larger ones  ), which is why the outboard weight is included in the CE maximum load. If you would add a 100 HP outboard, the heavier engine would reduce the crew carrying capacity to 4 rather than 5-6.
[/quote]

I'd forgotten all that CE plate nonsense....I've never been convinced...
 Is the BC 23 not plated for six?
Six "IMO/UKMCA official/standard adults"  at 75kg/head, even with only 5kg of personal effects comes to about 480kg. About my half tonne.

[MarkDarley]

https://www.bhg-marine.co.uk/yamaha-f6cmhs-6hp-standard-shaft-outboard-39-p.asp

I bought this for my BR20 and find it adequate for the BC23 though very noisy. As far as I am concerned it is also quite heavy enough!
I believe part of the noise issue, possibly a large part, is due to the drum like effect of the engine well.  I have purchased a couple of closed cell yoga mats which I plan to spray white, and velcro to the vertical sides of the engine well in an attempt to dampen the noise.

Normally I use my Epropulsion but it is totally inadequate in a headwind in confined quarters due to the larger windage of the BC23 rig.  When cruising in confined tidal waters I intend to carry the 4 stroke and put up with the noise when I am forced to use it.  During the fuel strike in France in October, I was lucky I had packed both as we were unable to buy fuel for the four stroke!