New Trawler Forum member, Hello everyone, OA mk1 question!!!!

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Jan D

Member
Joined
Nov 18, 2023
Messages
10
Vessel Name
SeaDuction & Karis
Vessel Make
Mediterranean 38 & Ocean Alexander 50 mk1
Hello trawler forum members. I have definitely enjoyed a long time of reading posts on here via my husband and finally became a member myself.

We are looking for our next vessel and have narrowed our search to just a couple of boats we both like.

We currently have a 1990 Mediterranean 38 and have been chartering with it for 12 years. We are looking for a larger vessel, a liveaboard, next.

I have a question. Does anyone have the correct gross tonnage for an OA mk1 (not extended, just the 50') ? We looked at one this week and the documentation paperwork was on the boat, but it listed it as 23 and that's what our mediterranean is, which is a smaller vessel, so that's incorrect. (Sadly, Our current vessel has incorrect GRT on the form also). Must be a common mistake. If no one knows, could someone tell me the depth of the vessel from gunnel to keel midship and I can calculate it so we know. If definitely affects my masters tonnage rating so I want to get it correct the first time on the paperwork if we purchase the mk1 we looked at.

Thanks everyone. I look forward to reading more posts here. Maybe one day I'll be able to contribute to your questions, but for now, I enjoy reading your input. What a fabulous collection of knowledge on here.
 
Welcome aboard. The tonnage is a calculated measurement and it can easily be done incorrectly. But if the boat meets your needs I wouldn’t be concerned about the tonnage.
 
Thanks for the welcome.

Yes, I know the formula, but the tonnage does affect my ability to upgrade my masters tonnage rating, if it's incorrectly low. Yes, it's a calculation of volume, not displacement. I just need the depth measurement to do the calculation so it's correctly submitted when we change the documentation to us if we purchase this vessel. Otherwise, I wouldn't care about it, agreed. But it does affect us because of the need for grt to upgrade our licenses.

Thanks... hopefully someone can tell me. Otherwise, I'll have to have the broker measure this vessel since if we purchase it, we won't be there until after the paperwork has already been submitted (it's not a local vessel for us).
 
When I purchased my OA Mk1 50 back in 2012 the surveyor listed the following:
Gross tons 38. Net tons 31
Length 50 Breadth 15.5 Depth 7.5

Not sure where the info was sourced, but the boat was USCG registered at that time.

Hope this is helpful.
 
Hmmm, that gives you 3 feet of freeboard from the waterline up to the gunnel midship. That is probably not correct either. The draft is 4.5'. Interesting how many boats have incorrect GRT listed on the documentation. Thanks so much for posting that.
 
Hmmm, that gives you 3 feet of freeboard from the waterline up to the gunnel midship. That is probably not correct either. The draft is 4.5'. Interesting how many boats have incorrect GRT listed on the documentation. Thanks so much for posting that.

Just got back to my berth and checked. Midship waterline to top of caprail is 1.45m.

Your draft figure is for aft, near the props, and is basically controlled by the keel which gets deeper as you go aft and although I've never measure, it would be of the order of 1'6" below the hull at that point.

When out of the water I measured my transducer depth below water line and it was approx 1m. They are on the hull, keel is deeper even there, about 1/4 way aft. Midship will be a bit more, but nothing like your 4'5" overall draft figure.

In conclusion the 7'6" depth I quoted above is likely correct. It was not made up, it was sourced from somewhere. I just don't know where.
 
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OK so I just remembered that my Australian Ship's registration had the numbers you want. I put them on the application form back in 2012 having read them off the USCG registration.
Length 15.24m Max breadth 4.72m Moulded depth amidships 2.28m

I'm curious as to what your formula calculates Gross tons based on those figures....
 
Oh good to know. Thank you. Much appreciated. jan
 
OK so I just remembered that my Australian Ship's registration had the numbers you want. I put them on the application form back in 2012 having read them off the USCG registration.
Length 15.24m Max breadth 4.72m Moulded depth amidships 2.28m

I'm curious as to what your formula calculates Gross tons based on those figures....

For power boats: Gross Tonnage = (.67 x L x B x D) divided by 100.

it would be 38.9 (definitely not 23 as indicated on the documentation form on the vessel we just viewed). Thanks... that is good to know. jan
 
The Mk 1's are often quoted as having a displacement of 48,000 lb. Someone not knowing what they are doing might think thats near 23 gross tons, but of course gross tons and displacement are two different things.

Also, I believe that even when delivered new (lightship, no fluids) they would have been more than 48,000 lbs. My displacement is typically 30 t when hauling out, at many different yards over the years. Yes, I've usually had some fuel (but not a huge amount) and water on board at the time.
 
Good afternoon from Greece, welcome aboard.:thumb:
 
The Mk 1's are often quoted as having a displacement of 48,000 lb. Someone not knowing what they are doing might think thats near 23 gross tons, but of course gross tons and displacement are two different things.

Also, I believe that even when delivered new (lightship, no fluids) they would have been more than 48,000 lbs. My displacement is typically 30 t when hauling out, at many different yards over the years. Yes, I've usually had some fuel (but not a huge amount) and water on board at the time.
Thank you. Yes that was my thought as well. I really appreciate the welcomes and quick replies solidifying my thoughts and the correct info to place on our documentation paperwork if we are successful in purchasing this vessel.
 
Tonnage

We have a 1980 50’ OA RPH. Last time we were out on the slings we weighed in at 51,000 lbs. Not fully fueled or water. That would be I think at~25 gross tons.
 
We have a 1980 50’ OA RPH. Last time we were out on the slings we weighed in at 51,000 lbs. Not fully fueled or water. That would be I think at~25 gross tons.

The OP was interested in gross tonnage as relates to a masters license. In that case tonnage is a volumetric measurement not an actual weight measurement.
 
Just got back to my berth and checked. Midship waterline to top of caprail is 1.45m.

Your draft figure is for aft, near the props, and is basically controlled by the keel which gets deeper as you go aft and although I've never measure, it would be of the order of 1'6" below the hull at that point.

When out of the water I measured my transducer depth below water line and it was approx 1m. They are on the hull, keel is deeper even there, about 1/4 way aft. Midship will be a bit more, but nothing like your 4'5" overall draft figure.

In conclusion the 7'6" depth I quoted above is likely correct. It was not made up, it was sourced from somewhere. I just don't know where.


You did the metric/imperial shift quite a few times here Brian :)
 
You did the metric/imperial shift quite a few times here Brian :)

:D
Apologies for any confusion!
But, you're probably about my age and grew up here in Australia with the change to metric and can subconsciously convert as well. :)
 
:D
Apologies for any confusion!
But, you're probably about my age and grew up here in Australia with the change to metric and can subconsciously convert as well. :)

Ha, yes, exactly, but it’s still amusing when others do it, I do it all the time. My 30 year old son has been doing the same thing all his life. The exposure to TV shows produced in imperial oriented countries certainly attributed.
 
:D
Apologies for any confusion!
But, you're probably about my age and grew up here in Australia with the change to metric and can subconsciously convert as well. :)
Ever noticed tyre sizing? A blend of imperial and metric, eg.the rears on my car are 20(inches) x265 x35(both metric). Weird, been like that forever.
 
W hat I hate is when they use both SAE and metric bolts on one piece of equipment.
 
If the USCG documented tonnage seems to be in error, call the NVDC (National Vessel Documentation Center) and see what they have issued in the past for other vessels of the specs.
 
Greetings Jan D...........you've chosen a good boat to consider. Documented vessels can have incorrect details, and it appears the documents you are looking at may have some of those. The GRT on my Mk 1 (original 50') is 26. Of course variables of these ratings are certainly based on design, powerplants, gen sets, water and fuel tank levels, etc.
If I may ask, which MK 1 (boat name) are/were you looking at?
 
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