economical coastal cruiser

[Brian@BNE]

Will - points taken, but of course its capital cost and weight/size penalty as well as operating cost.

Oyster - quite right in that outboards are likely to offer the best solution. But i'd really like to get away from petol. And i think we're heading for almost double LOA, which will inevitably fill up with stuff. So lightish boat, no 8" draft but i'd think much less than 3' also. What is your beam? Any photos or specs?

[Guest625101138]

Originally Posted by Willallison

I'm not going to re-enter the to-and-fro foolishness of a few days ago - I still have a headache from bashing my head against that (very thick) brick wall... but on the prop front, I have just two questions... how does a 900mm prop not impinge on a 550mm draft? Or is the operator required to keep the prop blades horizontal?
....

Anyone familiar with high performance trailerable yachts readily accept the idea of lifting a keel that weighs maybe half the boat displacement so the boat can be beached or trailered. I don't think it is overly demanding to expect to place a 2-bladed prop horizontal for the same purpose. No doubt there are other options worth working through if it is indeed an issue for the final solution.

We are at a point where the weight and or windage has to be reduced to achieve one of the prime objectives. I don't think reducing weight below 3t will be challenging.

To achieve the 12kts/12nmpg (for US gallons) limits input power to about 16kW. So the optimised prop to achieve this will likely come in smaller diameter than 0.9m.

Rick W

[Oyster]

Originally Posted by Brian@BNE

Will - points taken, but of course its capital cost and weight/size penalty as well as operating cost.

Oyster - quite right in that outboards are likely to offer the best solution. But i'd really like to get away from petol. And i think we're heading for almost double LOA, which will inevitably fill up with stuff. So lightish boat, no 8" draft but i'd think much less than 3' also. What is your beam? Any photos or specs?

Well to enjoy the eastern seaboard and do it in a trailable boat, there are severe restrictions without petrol. There has been several discussed here by some highly qualified professionals in this thread. My response was targetted. So I will not attempt to reinvent the wheel. I know every single inche of the waters from the gulf coast to New York, and have lost nothing outside of the sea buoys any more. So this was the reason I stuck to a really shalllow but managable and marginable draft in a wharped bottom design for decent handling while transitting the semi open waters and ease of loading and launching too.

[Willallison]

Originally Posted by Rick Willoughby

Anyone familiar with high performance trailerable yachts readily accept the idea of lifting a keel that weighs maybe half the boat displacement so the boat can be beached or trailered. I don't think it is overly demanding to expect to place a 2-bladed prop horizontal for the same purpose. No doubt there are other options worth working through if it is indeed an issue for the final solution.

We are at a point where the weight and or windage has to be reduced to achieve one of the prime objectives. I don't think reducing weight below 3t will be challenging.

To achieve the 12kts/12nmpg (for US gallons) limits input power to about 16kW. So the optimised prop to achieve this will likely come in smaller diameter than 0.9m.

Rick W

Fair point - though of course in this case you suddenly won't have an engine....

Given that we don't have a list of the stuff to go onboard - except in the broadest sense, how are we to determine the displacement? As I keep saying, without the SOR, all of this is meaningless. Sandy has already said he wants aircon. That means a genset. There will probable need to be at least 200-300AH of batteries. The head / shower will in many places require the inclusion of black & greay water tanks. So the list goes. But until it has been compiled, hullforms and powering requirements are of no value beyond entertainment

[Brian@BNE]

Oyster - ok, and i'm assuming i'm not the target as i've felt no pain yet. but your beam is? construction materials? Photos to share?

And a chance for me to clarify - not double your 24', but to an LOA up to 11m or 36'.

Rick - prop horizontal for transport is fine with me anyway, i've scraped outboard skegs on kerbs in the past. Obviously it has to drop down for running? With the greater prop efficiency comes smaller motor, so is the guess of $5-10k for the drop-dpwn solution more than will be needed?

Will your stabilisers take beam off the main hull (to stay trailerable etc) or are they demountable, perhaps carried adjacent the bow while on the trailer?

[Guest625101138]

Originally Posted by Willallison

Fair point - though of course in this case you suddenly won't have an engine....

Given that we don't have a list of the stuff to go onboard - except in the broadest sense, how are we to determine the displacement? As I keep saying, without the SOR, all of this is meaningless. Sandy has already said he wants aircon. That means a genset. There will probable need to be at least 200-300AH of batteries. The head / shower will in many places require the inclusion of black & greay water tanks. So the list goes. But until it has been compiled, hullforms and powering requirements are of no value beyond entertainment

Outboard manufacturers would make a bundle of money if outboard engines were destroyed every time a prop hit the bottom or a log. The optimum design would surely take this as a standard requirement and do it quite a bit more elegantly than what is done in a typical outboard.

We were given a displacement of 3t to work with in the first iteration. It is more than I would want but the customer is always right. He now has a bit more information to refine his requirements. Having got to where we are I can say with some confidence that a diesel powered 3t boat of 12m length is very unlikely to achieve 12nmpg in 20kts of wind and associated see conditions if the superstructure is 3m high and beam is 2.5m.

So the design space needs to be changed. My first place would be to revisit the weight estimate. We already have some reasonable data for the hull and powering to base that on now.

I can also see possibilities for reducing the height of the superstructure.

I am happy to entertain those that want to know more about the design space. No doubt there are some who know all this and have already built an efficient coastal cruiser capable of 12kts at 12nm/USG for their own use but others need to start somewhere. It only takes a few minutes of my time and is a diversion from other more boring things that people pay me lots of money to do.

Rick W

[Guest625101138]

Originally Posted by erik818

.....

What we’re doing now on this thread is what I would call a concept study. We do them quite often where I work, financed from the marketing budget, R&D budget or by a customer. A detailed SoR isn’t needed for such a study; the SoR can be the outcome of the study.
For a concept study, I think we’re attacking the problem in the right way by making a design assumption and then investigate the consequences.

Erik

In my working world we call it a feasibility study. And this study is unfinanced - just a diversion.

In a post or two back I have stated that a diesel driven 3t/12kts/12nmpg/12mlong/3mhigh/2.5mwide/20ktswind-waves is not feasible unless you approach a divine order who can change the laws of physics.

We are not far from it being feasible. I assumed USG rather than Imperial as well and nm rather than statute. With Imperial we are close but there are things you may not like in the current configuration.

Rick W

[Willallison]

Ask most designers and I'm pretty sure they'd tell you that the customer is usually wrong - just that it's not always prudent to tell them so

I'm intrigued about the prospect of a setup the provides protection to a prop that could be as much as 1.7m below the surface (550mm draft, 20% tip clearance, 900mm diameter). Any idea's on how you would actually do this?

[Brian@BNE]

Rick -in this part of the world, above 8m and used inshore, we will need grey/black water management of some kind and even if not mandatory elsewhere it is the responsible thing to do. So more weight and some internal volume gobbled up.

I agree with weight of boat <3t, and aim is as light as practical. But the 'ton of gear' as well as supplies & people will be pretty real and not all metaphorical. Personally i'd have height closer to 2m than 3, and beam no more than 2.2 but realise, perhaps i'm still dreamin' about what can be achieved. I've had a lot more than 2 cents worth today so will now (try!) to wait and see where Erik and fcfc want to steer discussion from what has been put forward today. I'm very interested but not yet committed and i think they are likely to start a build before me. I also want to see what comes out of the 'Competition II' thing sometime after mid-April before setting anything is stone (or expoxy).

As to SOR, I'll have everything that will fit in the external dimensions that now seem fairly well constrained, please. A marginal improvement of telling the designer i want a full condo at the outset. I'll leave the aircon for the next stage (bigger boat) and am slowly 'being educated' by the designers about what will add too much weight or height and has to be ditched for economy on motor size and fuel consumption. But i'll likely trade-off fuel economy eventually (but not the 12k speed!) after considering what perhaps surprisingly small proportion of the Total Cost of Ownership per annum is taken by fuel. Some of this gets away from Sandy's original post, but it seems he is not following the thread now? Perhaps he is building the solution as we chatter? I don't think he indicated crew - i'm think of two adults and up to 2 kids for cruising, with capacity to take an additonal 4 adults on day trips only. The latter is intermittent but shouldn't cripple performance.

And i'm sure i'm not the only one to appreciate the input of all experienced builders and designers contributing here in particular, and glad to hear that it isn't too much of a hassle or too time consuming for you guys.

[Willallison]

Brian - and when this 'concept' comes back and says to you - oh sorry, you can't take most of that 1 ton of gear with you, can't have a shower on board, can't go to 1/2 the places you want to because the draft is almost 6 feet ... what will you do....?
Why not approach this in the way a professional would... compile a detailed list of all the things you want on board, where you want to go, how far and fast, what you want it to look like, what you don't want it to look like, etc, etc.
Then - and only then - does it make any sense to go down the path of custom building a boat. Otherwise you might as well go buy yourself a Whittley or Bayliner. It will DEFINITELY be cheaper, you will probably get better resale value, and you could be on the water tomorrow....

[Brian@BNE]

will - Shower can be a pump and a self draining cockpit floor if too cold for a swim (or too scared of bull sharks in that location). Sure, no soap for those showers, so back to shore now and then and if not feasible then 'bird bath' with a bucket and down the sink is fine. But enclosed head might be ok for shower too, even though after a short time 'on our next boat ....better shower' will reverberate around the cabin.

My mission is to have something barely trailerable and just containerable to decide whether I want to do extended cruising bordering on liveaboard. Something like your K5 is certainly a consideration for that - not readily transportable. Yes i could buy a cheap one that size to try out for a while, but it would likely need some repairs, perhaps repowering and new electronics and other gear. Would the post-purchase repairs be greater than the trailerable boat? Could be quite easily. If in due course the decision is, no the extended cruise is not for me, then there is a lot more capital tied up in something that could be hard to unload without taking a big hit. And testing my enthusiasm for lots of time on the water with a heavily compromised largeish old boat could well predjudice the answer.

But you have got my attention. 2.2m beam, 11m LOA - that should carry a lot of weight and surely you are joking about the 6' number.

The trouble with approaching it 'as a professional' is that you end up with something that is quite likely already available on the market, which doesn't suit. I'll ignore the mention of Bayliner, some bait just isn't attractive enough to take a bite. Different outcomes can arise many ways but different approach and thinking aloud here looks to be working.

[Brian@BNE]

Originally Posted by Willallison

Ask most designers and I'm pretty sure they'd tell you that the customer is usually wrong - just that it's not always prudent to tell them so

I'm intrigued about the prospect of a setup the provides protection to a prop that could be as much as 1.7m below the surface (550mm draft, 20% tip clearance, 900mm diameter). Any idea's on how you would actually do this?
__________________
.

will - even if there 500mm draft at the finish of this, back where the prop is the hull should/could be higher surely?

ok, i can't stay away - this place is addictive!

Brian Eiland shows a belt drive concept for a yacht, which is similar power http://www.runningtideyachts.com/power/

It might only be concept rather than reality somewhere. In one place he shows uni-joint shaft connected to a fixed engine, but i think somewhere else he may have shown the whole motor/drive lifting up. Perhaps that is not too good for stability though unless we do end up with a big battery bank! I suppose small pod drives, if raisable, would also work. The motor might not have to be lifted more than say 450mm (shallow water & slow speed travel on water, could be higher for on the trailer) and if located in the cockpit area then temporarily putting up with it would be OK. For those folk in the ICW who apparently have to (or want to) deal with long distances of quite shallow water, then a smaller prop, larger motor (or even the trim/tilt outboard) might be better, i accept.

[Guest625101138]

Originally Posted by Willallison

Ask most designers and I'm pretty sure they'd tell you that the customer is usually wrong - just that it's not always prudent to tell them so

I expect that the designers with this view not inundated with design briefs.

Originally Posted by Willallison

I'm intrigued about the prospect of a setup the provides protection to a prop that could be as much as 1.7m below the surface (550mm draft, 20% tip clearance, 900mm diameter). Any idea's on how you would actually do this?

I am intrigued why you think the prop needs to be under the deepest part of the hull?

Another little gem of common wisdom that needs to be dispelled is the idea of a 20% of diameter clearance. A large diameter prop with high blade aspect achieves high efficiency because there is low velocity ratio and low induced drag. The extra 10kW that is wasted in the 0.3m prop is creating a huge amount of unwanted turbulence that will impact on anything near it. It needs at least 20% of diameter clearance. A large diameter efficient prop can have less tip clearance for the same impact on the hull.

The draft to swing a 900mm prop is about 1m. It could be reduced by going to a tunnel. The static draft without doing anything fancy like folding or elevating is the draft of the hull. That is why I limited consideration to 900mm. Anything deeper will impose extra drag to support it that is going to offset the small gains in efficiency above this diameter.

In any event these questions do not need to be resolved yet because we know there is no solution possible within the current design space.

Rick W

[Willallison]

Originally Posted by Rick Willoughby

I expect that the designers with this view not inundated with design briefs.

Rick W

How about I start a thread and we'll ask shall we...?

The client is always right...?

Can you point me towards the research that suggests that a large, slow-turning prop needs less than 'normal' tip clearance?
And how would you propose to deal with the rythmic vibration often caused by insufficent tip clearance - and which is worsened by having a two-bladed design?
As to the draft, you will note that I said as much as 1.7m But even allowing for placing it under possibly shallower aft sections it is still likely to be approaching 1.3m.
I guess you could put it in a tunnell, the top of which is marginally above the waterline - as was done succesfully by Aitkin with his skiff - but imagine the size of the tunnell to allow for such a big prop...! Sorry Brian... there goes your cockpit!

And now, I really hate to bring this up, BUT....

The draft to swing a 900mm prop is about 1m. It could be reduced by going to a tunnel. The static draft without doing anything fancy like folding or elevating is the draft of the hull. That is why I limited consideration to 900mm. Anything deeper will impose extra drag to support it that is going to offset the small gains in efficiency above this diameter

Wouldn't this suggest that you compromised on the most efficient diameter for the prop for the sake of lower draft...????

[Guest625101138]

Originally Posted by Willallison

..Can you point me towards the research that suggests that a large, slow-turning prop needs less than 'normal' tip clearance?

I doubt that you will find this written up because there are no marine props designed for such low blade loading. However if you sit down and do your own analysis of the blade loading you can approximate the local tip pressure and you will see why you do not need a large tip clearance.

Originally Posted by Willallison

And how would you propose to deal with the rythmic vibration often caused by insufficent tip clearance - and which is worsened by having a two-bladed design?

If the pressure disturbance is small the force exhibited on hull panels is small. It is also lower frequency so not as hard to stiffen the panel beyond harmonic range.

Originally Posted by Willallison

As to the draft, you will note that I said as much as 1.7m But even allowing for placing it under possibly shallower aft sections it is still likely to be approaching 1.3m.
I guess you could put it in a tunnell, the top of which is marginally above the waterline - as was done succesfully by Aitkin with his skiff - but imagine the size of the tunnell to allow for such a big prop...! Sorry Brian... there goes your cockpit!

And now, I really hate to bring this up, BUT....


Wouldn't this suggest that you compromised on the most efficient diameter for the prop for the sake of lower draft...????

Not at all. Prop efficiency is only one factor in the overall design space that we have to optimise within.

After almost a week now you still do not understand the difference between compromise and optimisation. The distinction is something all designers should have an acute appreciation of. I have attached some definitions for your education.

One involves giving-in or giving ground while the other is a search for the win-win outcome that goes to the best solution - of course for a complex design it is only the best solution in that time frame because there will always be unknowns and better ways in the future. This is the basis of continuous improvement. But then if you already know everything then the journey has ended.

I can see compromise pervades your thinking. You have already placed the customer in the "dumb git" bin who you are going to spend a lot of time bringing around to your way of thinking because you are always right and he is always wrong.

I my world the designer enters a partnership with the customer to go on the journey together, learning from each other to find the most satisfying solution - the optimum.

In past lives, for my paid work, I have been the client representative and I have invariably found the engineers and senior personnel in the design team have great respect for the knowledge of the customer. They spend a lot of time seeking out why there are certain preferences and objectives and explaining how these impact on the result. They provide the basic input for making value based decisions. They get to know the economic drivers of the customer's business reasonably well.

Rick W