Extending a Mk 1 from 50 to 55 ft

Hello Almad. Hopefully you are still engaged in this topic. I just purchased an unadulterated '78 OA 50 Mk 1. She is in pretty much pristine shape as far as boats go haha. I am looking at adding passive stabilization, a hull extension and maybe a bulb. I have been a commercial fisherman my whole life here in British Columbia now retired as of July. Of course it is harder to justify spending money on a pleasure boat. She is just fine now as long as its relatively calm. How was your passage home and how are you liking the attributes of your vessel?

IMO, extending the transom five feet should only be done if one repositions the propulsion. At an absolute minimum, the rudders though I would suspect in order to work properly the prop needs to be further back as well.
One of our club members had an extended stern. The rudder and props were not relocated. It handled terribly!
There’s a reason the rudder and props are located aft and not amidships!
Proceed carefully.

cks said:

IMO, extending the transom five feet should only be done if one repositions the propulsion. At an absolute minimum, the rudders though I would suspect in order to work properly the prop needs to be further back as well.
One of our club members had an extended stern. The rudder and props were not relocated. It handled terribly!
There’s a reason the rudder and props are located aft and not amidships!
Proceed carefully.

Click to expand...

What was the brand and length of the boat your club member had? How long an extension?

The OA Mk 1 was designed by Ed Monk Jr. He also designed the 5' extension early on.

There have been numerous done - in the range of 20-30 I would estimate. As far as I know no-one has moved props or rudders, and I've not heard of any handling issues at all. Now, some other brands/hull designs might well have a different outcome.

Thanks CKS. I have spoke with three OA 50 Mk 1 owners with extensions and been onboard two of them. Both that I was aboard had not moved rudders or propellers. They both handled without any noticeable difference. The third one did have the propulsion moved aft and it also handled well. (He owned the vessel prior to the refit)

But the OA 50 Mk 1's are built with the propulsion as far aft as possible. So this is probably alleviating the issue. Thank you for your info; the more the better : )

Hello Aquabelle. Are you still active on this site?
Rgds, Almad

Hi Almad....I'm still alive and kicking. PM me if there is something you think I could help you with

Gentlemen.
Has anyone had any experience with the stability or righting ability of mk1’s.
I frequently read the mv Dirona blog, which is a 52’ Nordhavn, and they have noted that on no less than our very own Mad Mile (sth end of Fraser Is Qld) that they recovered from a nock of around 60 deg!
This stretch of water is well renown for its beam seas and not the kind of place you want to take on in a sth swell.
But 60 Deg??
Anyway, I have been talking to a surveyor recently regards establishing cg, downflooding etc etc, but it would be great to hear if anyone has done any homework on this?
Thx, Alan

Alan, our ER vents are one of the biggest concerns for down-flooding. Second would be the large and weak salon windows. I doubled up on the vents, with another 4 on the inside of the bulwark, opposite those on the outside. I have hard covers for the outside ones that can be fitted, two metal-threads hold each one in place. The theory was that it would take a bigger roll, or higher wave to come over the bulwark. But I've never put them to the test!

For Dirona, it was not in the Mad Mile that they had the big roll. They broached before even getting to the entry waypoint for the bar. So they had not crossed the bar or entered the mad Mile. They either had not taken the bar seriously enough (a stiff southerly had been running for quite a while) or were over-confident in their boat. They survived, but with some damage.

Hi Brian.
That’s good info. So, just to confirm, the ones on the inside of the bulwarks are still above deck height, which are then turned down, protruding into the salon area, penetrating through the floor of the salon into the ceiling of the engine rm?
I had thought of doing something similar, but building out the external thickness of the salon wall to accommodate the vents, locating them in the vertical face of the superstructure just below window sill height. By doing this I could also beef up the wall strength between the windows.
I would lose approx 3”/75mm of the side deck, but I think this is one solution to two problems.
If anyone understands that explanation, you deserve a medal, but would also appreciate your thought on that or any other issue to do with making our babies more seaworthy.
Alan

I'll take some pics and then provide a better explanation (hopefully) tomorrow. It is not what you have been thinking....

By the way, what's the name of your OA?

Alan
I later realised that while I was describing vents on a 'normal' Mk 1, you have a flushdeck. We have the same hulls with vents on the hulls in the sme places But you can see your vents from the ER when standing, at about head height. For me, the bulwarks are hollow at least in that location and are higher than my ER ceiling.

First pic is starboard side rear exterior hull. I have Vetus vents, and we built a dorade internally for them, hence the 1/2" black spacer. The second pic is the port side, interior of the hull bulwark with a cover in place. The covers are not watertight, but if in open ocean I could have them on all 4 external hull vents and block most of the water from entering. But I would still have the same vent intake capacity as I originally had using the new vents added inside the bulwark.

If you are contemplating crossing oceans then I think you would be better to engineer some intake vents from much higher, perhaps on the boat deck. It will steal interior room, and be a lot of work. Good luck with your investigations - keep us posted!

Thx Brian. We inherited Pasha of Hamble.

Thx Brian. It really helps seeing where others have gone before.
Appreciate the logic you’ve used, and yes, even though a flush deck, will be applying the same.
Some thing for thought. Looking at the nordhavns, the weight of comparable sized boats are around double ours. I assume this has got a lot to do with underwater ballast, so am trying to get my head around this logic.
Coming back from 60deg would have to have something to do with this.
Do I add more weight to the keel
It’s worth taking the time to look at Cruising Sea Venture on YouTube.
They r up where you bought your boat from and they have just added a bulbous bow.
So many opinions on this, but they do it really well.
Rgds,
Alan

Well, very nice inheritance I must say!

You may already know all this (if so forgive me!), but in case you don't:

Nordhavn's are displacement hull designs, whereas the OA Mk 1 is a semi-displacement hull design. Nordy's have deeper draft & heavier construction in addition to some ballast. That gives them great static stability, but those factors and their hull shape makes them slow. They can't get over hull speed. But they will have better fuel efficiency and for some purposes, such as crossing oceans, they excel.

The Mk 1's, like other semi-displacement hull forms, can exceed hull speed given enough HP. Even modest use of the installed HP will be 1 or 2 kn faster than a displacement hull form without consuming prodigious quantities of fuel. They are less fuel efficient than both displacement hulls and fast planing hulls, but the compromises give a boat that is great for coastal cruising and typically has a lot of home comforts. Hence their popularity despite some shortcomings compared to the more specialised hull forms.

It won't be feasible to add ballast to a Mk 1 to mimic a Nordhavn. It would be like taking a sports sedan and converting it to a camper van. You might well succeed to some degree, but the cost in time and money won't make sense. And at the end of it you definitely would not have a sports sedan anymore, and likely will have poor camper as well.

In terms of cost to make a Mk 1 seaworthy for an ocean crossing, plus fuel etc for the crossing, it will likely be cheaper to ship it most of the way, or at least past long stretches of ocean or places prone to piracy. In your case some of the work has been done already, but doors and windows are huge vulnerabilities in addition to the ER vents. Sorting out those issues, in a way that does not butcher the boat, wont be cheap. Yes, you could though-bolt 12-15mm perspex over the windows and it might be enough, but then there is some repair work to do once it is removed at the end of the crossing which could be challenging to look decent. Of course by using Pilot Charts and travelling at the right time of the year, and with a bit of luck, you may very well be able to get the boat ready with relatively minor modification and cross without drama. I was intending to do that back in 2012 and started refitting the boat to be fit for purpose. But then shipping rates halved and it became the prudent option to ship.

These days I regard my boat as an outstanding vessel for long distance coastal cruising. It can handle more in terms of sea state than I am comfortable with. Crew sea sickness and fatigue can lead to adverse consequences from poor decisions, even if the vessel is fine. At this point I don't think I would ever cross an ocean in a semi-displacement hull because I don't have the tolerance for extended periods of being uncomfortable and just not enjoying it.

Having said all that, I hope you keep us posted on your research and plans. You might prove me wrong, and If so I'd be delighted to welcome you ashore at the end of a crossing!

Thx Brian.
Appreciate the feedback.
It all helps on the homework.
Can only dream of shipping rates halving.
$100kaud back to Aus and half the trip gets a 10% discount!