Power to weight ratios

[whisper]

The scenario is that the boat weighs approx 1350kg. excluding engine.
Propulsion options on offer are:-
a) Volvo diesel AD 31 with duo prop - 144hp at prop. max revs 3900 - 4100
Weight - 443kg incl. leg.
b) Mercruiser 1.7 TDI with Alpha drive - 117hp at prop max revs 4000 - 4400
Weight - only 300kg incl. leg.
We are worried about whether b) will be man enough to power the boat with 4 up plus kit. Whether it will be noisier at cruising speed of 18 - 20 Knots as it will presumably be having to rev higher. Whether handling with Alpha drive will be worse than with Duo Prop. Obviously b) is a much lighter combination.
Finally is it likely that the speed in knots at tickover will be higher with b) rather than a)?
There - that's something for you intelligentsia to direct yor grey matter at or should I post this with PBO?

 

[duncan]

whilst hull design, waterline length and potentially trim tabs all have a function the ration of 16 kg/hp is often quoted as an indicator of planning ease for outdrive/outboard powered craft(shaft drive helps a fair bit and even 20 works apparantly).
If you quoted weight is based on boat alone then adding only 400kg for people gear and fuel has the smaller unit at 17 and the more powerfull one at 15.
Different engines have diffent 'noise charastics, and the smaller one allows more soundproofing room!
tickover speed will depend on tickover revs and props rather than drive alone
overall driving with th eduo props is a better experience - trim effect on steering goes away, and you will get a better, more efficent, power transmission over all.
The newer Merc diesel may well be more fuel efficent in itself but doesn't look as if it is right for this craft - especially if you anticipate carrying more than a couple of people and light gear around most of the time.
all of course imho - workings given so that people can argue about the basis rathert han just the conclusion!
good luck
ps it would be usefull to note the boat itself - what is it?

 

[whisper]

Thanks Duncan, I assumed that the lack of response to this posting meant that it was far too technical for this forum's users!!

Re the boat model, it will be - I hope - a Ryds 23 from Sweden. It is a Walkaround type of sportsboat with a small cuddy and will be used in S.Devon & Cornwall for dayboating and occasional overnighting.

This would be the first of this model to enter the UK, there is no active dealer here, so it will be a "personal import". We tested and were about to order the Volvo powered model after more than 3 yrs of investigating boats from Beneteau, Faeton, Jeanneau, Ocqueteau,Quicksilver,Rodman,Orkney etc.etc. In our opinion the build quality is way ahead of all of these apart from the Orkney range which didn't comply with some of our other requirements.
As mentioned in my original post, Ryds have for some reason decided to discontinue the Volvo and have replaced it with the Mercruiser along with a considerable price hike. I have asked them why but they are not forthcoming.

Thanks for your time.

 

[tr7v8]

List price for the two engines is pretty much the same so their shouldn't be a difference from that. I believe that V"£v& give bigger "builder" discounts than Mercruiser but that shouldn't be that vast.
My personal opinion is that the bigger engine would be better once you start loading it up, as it'll have greater torque. However the trade off will be that the V*l^o is older technology than the Merc and that'll reflect in fuel consumption etc.

Jim
---------

 

[duncan]

no probs
went through similar - 17ft dory (OB)-> 21ft walkround cuddy(OB) -> 25ft HT walkround (diesel OD)
I am sure RYDS were active in the UK a couple of years ago - remember looking at them at the London show; maybe the influence of Guiness confusing my memory.....
I am sure the Merc will do the job but would personally be looking for around the 150 hp mark if it is a fully planning hull design with an expectation of around 26knots top end / 22 knots cruise.
as you can see I am a total walkround addict - great family / fun / fishing boats
spent a week on the 21ft'er in Plymouth back in '97 - 2 dad's and 2 sons though; but fun.

 

[whisper]

Totally agree about Walkarounds - safe, practical & fun. Have a look at www. rydsbatar.se where the 23 is featured. We would have prefered to have the open top version but can't find one with the Volvo in it . Have therefore gone for an unused 2002 hard top version with the V****. These have a decent sized sunroof so you are not totally enclosed should the sun come out. Hull is deep vee and trim tabs are standard.

Your boat progression has been similar to ours except for Dory substitute Orkney fishing boat. If we could justify the expenditure we would spend a lot more and buy a Botnia Targa 25. To us this provides the best of all worlds with Scandinavian build quality, very low depreciation, compact size, both open air helm and wheelhouse , reasonable accommodation, deep vee hull, diesel power and it's still a walkaround. Whoops, I must calm down, getting over excited!!!!

Your memory re Ryds is correct, they have had UK dealers in the past but haven't done so for the last 2 or 3 years at least.

Cheers

 

[duncan]

spooky - the Botnia Targa 27 was my first choice...

the 23 looks very pretty, and the HT not to imposeing. Also the walkround is more at floor level than inset in the deck as mine is.
It would be interesting to see how the financials compare to my Faeton when you are all done - appreciate a PM if you don't mind.

When are you planning delivery / use? If you are ever in thePoole area let me know and you are more than welcome to come out - fishing or family whichever suits.
regards

 

[adarcy]

Re: Torque though?

Haven't got any figures to hand but suggest you check the torque figures before deciding finally.
I believe but am willing 2 B corrected that the Merc TDI is new tech and MAY have much more torque than the older tech in the volvo. I accept that duprops are better and more efficient etc but it is torque that gets you up on the plane and keeps you there over the waves. So 150kg lighter = 2 people , quite likely to have better fuel efficiency against poorer prop efficiency and allegedly more expensive parts for servicing.

you pays yer money and ........

but I still suggest looking at the torque figures first, if it's quite a bit higher than the volvo I'd be tempted by the Merc

Anthony