Interesting New Boat splashed recently

[Stewey]

This is a very interesting thread on the build and launch of a new 21 ft boat. Whole thread contains all build from start to finish, however pages 98 to end are the most interesting. This is not an etec versus others thread, more an example that you need to match your hull, motor, gear ratio and prop correctly.

For those that dont want to read thread below, in short a Suzuki 140 was fitted originally. 3500 rpm gave 13.5knots, wide open was 32 knots, gear box ratio 2.59/1. Owner was not happy with this as hull specs showed a better result should have ben obtained. Motor was changed for a 150 etec. Now 3500rpm was 28 knots and wide open in 25kn plus head wind and crap seas was 44 knots rising. It would appear that fine tuning of the prop will achieve a better result again. Both engines were fitted with solas props suited accordingly. Etec gear ratio 1.89/1 i think. Les than 2/1.

I do not pretend to understand the mathematic calculations involved in obtaining these results, but they are impressive.

http://bowdidgemarinedesigns.com/For....php?f=18&t=55


This is not an ad or endorsement and not posted to start any us versus them arguments, just impressed me and found it interesting enough to share.

Regard

Stewey

[ozynorts]

love those Bowdidge designs.

[Stewey]

Yup. His designs are eyecatching.

[finga]

Now that is a boat.
How nice would have looked if all the timber work on the hull was allowed to show through the glassing job?

[Stewey]

That boat would look good as timber but Dean's finish is brilliant. Awesome looking boat both on and off water.

I have just got back from a trip to brisbane to pick up my ply and glass etc for the SeaStrike 18 i am about to build. Wish that i had the experience and time to build the pro tournament. Maybe next one. But very happy with the Ss 18.

Also an absolute pleasure to deal with Mark Bowdidge. His knowledge and accessability are amazing. Nothing seems too hard for him or his patience.

Regards

Stewey

[stevemid]

Stewy yes, very very very interesting. The designer seems to say what really made the difference was the 2.59:1 gearbox ratio on the 140, vs the 1.86:1 on the 150. He said:

..." Actually Bounty Hunter, what your describing above, in regards to Deans boat or many other boats out there is not the fact of being under powered at all (although some are), but choosing the wrong outboard that has the wrong gearbox ratio to suit your boat. That's where it all comes down to. You need to match the speed/weight of boat/ HP/ gearbox ratio/ prop diameter and pitch. Without matching all these, your going backwards. Going from 140 HP to 150 Hp, doesn't give a huge difference in speed. Horsepower is horsepower. Whether your outboard is a "big" litre 140 or a "small" litre 140 , it's the same 140 hp. What does make a difference is matching that Hp with the correct gearbox ratio and prop. That's where the fuel savings comes in, the motor isn't working hard and your getting far better speeds.

Im wondering how do I make the correct motor decision for the boat Im building without going through an (2) engine swap to get to the right outcome?

[Stewey]

Stevemid, there is a plethora of calculations that mark goes through to obtain optimal gear ratio/motor size/ prop etc. To me it is all a bit of black magic:-:-):-)

For obvious reasons his calculations are proprietry information which are not freely published, and are different for each hull. My suggestion would be to contact your boat designer who should be able to guide you. Solas use the same calculations for their prop matching.

Sorry that i could not help more.

Regards

Stewey

[chris69]

hi there stewy yes its been a real eye opener for everyone but with mark haveing designed the boat he knows the weight and dynamics of the hull he designed and from drawings and calculations with the right motor and gear ratio and prop with the right diamter to pitch ratio,if all the boat builders and designers matched there hulls to motors with the right specs there would be a lot more happer boats out there with a lot more money in there pockets from the fuel saveings and a lot more faster boats,we are the lucky ones that have gotten the plans to build a great boat design and get all the support and tec info for it,its just something no other production boat builder does,to go from a 140hp to a 150hp with the right gear ratio and prop and get 12kts fast at top end 32kts to 44kts and and a 12% fuel saveing at 28kts is outstanding just by changeing the motor to get the right gear ratio as he calculated everything for,cheers chris.

PS Stewy theres no more skill needed to build the PT21 than the SS18, but once you have built your SS18 you will have no worries about the PT21 strip plank design.

[Swanie1975]

Great thread and awesome build. With regards to the motor would any make outboard around that 140-150 hp range been good so long as the gearbox ratio was around 1.9ish? What about the power delivery of a two stroke v a four stroke? Just curious as propping boats does seem to be a bit of a black art lol.

Cheers Ryan

[tunaticer]

Whilst I really like the hull design and the workmanship that went into it, I am left a tad flat with the sheerline of the gunnels. Straight flat tops do not make a boat appealing IMO. Apart from that it is a master piece and well worth presenting.

[Mister]

Arh the full circle, back to the grass roots when Harltey's / Fletcher's, Bowden & Wharrum were all the go but one difference now is material quality & differences.

Yet today some thing all this is cool and something different/

[johncar]

A nice build project indeed and in my favorite fishing setup being CC. Very nice classic lines with all the modern trimmings..
As far as the engine upgrade is concerned, although I don't doubt the recorded figures were taken in good faith and all that, Probably would have had similar results upgrading to any similar spec motors to the Etec 150. Don't really buy the drive ratio being so critical because correct prop selection should cater for any differences there. The Etec 150 is a considerably bigger CC engine and 6 cyl, there is much more to an engines spec than max HP or final drive ratio alone.
In saying that the Etec 150 or other similar 2 strokes may be more suited to this design than the heavier 4 strokes due to max transom weight considerations which is why I assume the owner chose the DF 140 initially rather than say a DF175 which the latter would seem more suitable for that size and weight boat if you want average to big top end speed.
So we shouldn't get into a 2 vs 4 stroke debate as there will always be niches for either so it seems, and from the figures this boat seems to love the 150 Etec being a good result for the owner.

[chris69]

Yes Swanie 150hp 2st the other 140hp 4 st but diffent gearing but different abilities to run a bigger pitch.

[chris69]

Tunaticer these strip plank boats are a great thing very strong as every 30mm there is a gap which is filled with bog and once glassed inside and out you have a composite contruction with a I bean every 30mm and the method of doing it is nothing like the old way this is way better faster and very easy.

[tunaticer]

Tunaticer these strip plank boats are a great thing very strong as every 30mm there is a gap which is filled with bog and once glassed inside and out you have a composite contruction with a I bean every 30mm and the method of doing it is nothing like the old way this is way better faster and very easy.

I agree with you on that Chris, I was not doubting its integrity or design, just the flat from bow to transom top of the gunnels.....the sort of thing you find on 3m punts etc, not usually on a superb boat such as this. A dressy curved sheerline would give it much more appeal than it currently has. The workmanship and design of the boat is excellent.