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Model profile · for Great Loop buyers
Mainship 390
What the published record establishes about this model, what owners report,
and the questions worth asking while a boat is still a listing. This page is about the
MODEL — a specific hull always gets its own homework.
Semi-planing 350/390. She wallows in a following sea and the integrated swim platform does not fix it — owners call it an annoyance, not a fault. No measured air draft exists for this hull.
The Mainship 350 and 390 are semi-planing trawler-style cruisers, popular enough on the Loop that resale is one of their real strengths — at a slight premium going in. Two things dominate the model layer, and neither appears on a listing. The swim platform is an extension of the hull rather than a bolt-on, and the originals were sealed foam with no inspection access, so they take on water at the rub-rail joint and lose buoyancy right aft. And she wallows in a following sea: the integrated platform was effectively the experiment that tested whether more length aft would cure that, and a former owner of the modified variant says it did not. Owners are consistent that this is a handling characteristic rather than a defect — fifteen years in one, jleonard's word is that they “just wallow a bit.” The standard mitigation on the smaller Mainship 34 is speed, but that fix is unreachable on the low-powered version of THAT hull, and nobody has tested the same logic against the 350/390 power options. What speed a given 390 actually makes is an open question per boat. Engine checks mean opening the salon floor, which is a livability question a survey will not raise. No measured air draft exists for this model, and there is no sourced material on the accommodation layout.
How to read this: 🟢 Fact = confirmed against a document, spec sheet,
or owner. Everything else — Claim, Pattern, Question, Context — is not yet
verified: things to check, not settled truth. Sources are linked where published.
Swim platform
CLAIM
The swim platform is a hull extension, and the originals flood
On the 350 and 390 the swim platform is an EXTENSION OF THE HULL — it displaces water and contributes buoyancy. So a platform full of water is not just weight far aft, it is lost buoyancy at the stern, which is why it moves draft and trim more than you would expect. The originals were foam-filled with no inspection access; water leaks in at the joint under the rub rail and saturates the foam. The foam slowed complete flooding but never kept them dry. Most have since been modified with access hatches, drains, or standalone bilge pumps.
src:
Gdavid (Mainship 34 MK1 owner, 390 operator), Trawler Forum, 2026-07-23
QUESTION
Has the swim platform been opened up with access and drainage, or is it still original sealed foam?
An original sealed platform is a latent water-saturation risk — weight, trim, and eventually structure. A retrofitted one has been addressed. A listing will never mention this and a survey has no particular reason to look.
src:
Gdavid, Trawler Forum, 2026-07-23
Your move: Ask whether the platform has been opened, and look for hatches, drains or a dedicated pump. If it is original, ask when it was last checked — and check the trim at the dock.
Handling
PATTERN
She wallows in a following sea, and the integrated platform does NOT fix it
Semi-planing hull; owners describe wallowing in a following sea. A Mainship 34 buyer asked whether adding an integrated swim platform would cure the same behaviour on his boat, and was told the 350/390 already ran that experiment. Gdavid: “it would increase the distance from the rudder to the back of the hull, decreasing steering effectiveness. This is basically what was done on the 350/390 and they don't handle any better in following seas.” Cartouche, who owned a 390 with the integrated platform: “it was still a bear in a following sea or a beam sea.” The platform looks like an improvement over the 34's bolted-on one, and on draft and displacement it is; on following-sea handling it is not, and it may cost steering authority.
src:
Gdavid (390 operator) and Cartouche (former 390 owner), Trawler Forum, 2026-07-28
CONTEXT
Read the wallowing as a characteristic, not a defect
The buyer who started that thread said the boats “struggle” and no owner agreed with the word. jleonard, fifteen years in one: “they just wallow a bit.” Jeff F: “ok in following seas.” Gdavid: “while it may be an annoyance, the boats have been in operation for close to 50 years now and they manage ok.” It is a handling characteristic with known mitigations, and the advice owners gave the worried new buyer was to go use the boat before modifying anything.
src:
jleonard, Jeff F, Gdavid — Trawler Forum, 2026-07-28
QUESTION
What speed does she actually make at cruise, and has anyone run her in a following sea?
On the Mainship 34 the standard mitigation for following-sea wallowing is SPEED — jleonard: “Speed cures that. If you can get it to 11 knots it smooths it out… if it's a 200 hp or more version.” And Jc180, who owns one: “the early MK I with the 160 hp engine is only an 8 knot boat at cruising speed.” So on THAT hull the standard fix is unreachable on the low-powered variant. Nobody has tested that logic against the 350/390's power options and we hold nothing on it. The 34's 11-knot threshold does NOT transfer — the 390 is a heavier, much beamier boat. What speed a given 390 actually makes is an open question per boat.
src:
jleonard and Jc180 on the 34, Trawler Forum, 2026-07-28; the 390 gap is an open question in the ledger
Your move: Ask what she cruises at, what she will do wide open, and whether the owner has run her in a following sea. If the answer is single digits, the standard fix for a known handling trait may not be available on this boat — ask about autopilot instead, which is what owners of the low-powered 34 fall back on.
Livability
CLAIM
Engine checks require opening the salon floor
True of both the 34 and the 390. You cannot do a morning engine check without asking everyone aboard to clear the salon and pulling the hatch. Gdavid names this as the thing he would most want to change about the boat. Good engine-room access generally needs a bigger boat. Worth actually performing a morning engine check before buying and deciding whether you would do it daily — it is a livability failure a survey has no reason to flag.
src:
Gdavid (owner of the 34, operator of the 390), Trawler Forum, 2026-07-23
CONTEXT
Much bigger than the 34, mostly on beam — and stairs to the flybridge
The 350/390 is a much bigger boat than the 34, primarily on beam, and the 34's storage is limited enough that living aboard it would be a challenge. Gdavid also notes the 350/390 has stairs to the flybridge rather than a ladder — “it will spoil you” and make it hard to go back. Minor, but a real ergonomic difference and invisible on a spec sheet.
src:
Gdavid, Trawler Forum, 2026-07-23
CONTEXT
Cruise-and-fish, not a fishing boat
Nice cockpit, and she will idle slow enough to troll for anything you would target en route — but the cruise speed makes a long run to a spot. Buy her to fish opportunistically while cruising, not to go fishing. Gdavid also notes the superstructure makes noise trolling across waves for extended periods.
src:
Gdavid (390 operator), Trawler Forum, Jul 2026
Draft and trim
CLAIM
Loading barely moves her draft — but confirm the number anyway
Gdavid, answering a direct question: “I doubt the draft is impacted very substantially when loaded down for cruising, the hull is quite wide and you would have to really load it down to see more than a couple inch difference.” The mechanism is hull form — a wide, beamy hull has large waterplane area and therefore high pounds-per-inch immersion, so a cruising load sinks her only a couple of inches. THIS NARROWS THE EXPECTED DELTA. IT DOES NOT REMOVE THE NEED TO CONFIRM THE FIGURE. The listed water draft is the design draft; the delta is a function of hull form, not a constant, and you do not know it until someone measures. Confirm the number — never assume it is worse, and never assume this claim means you can skip it. He also names the bigger variable: the swim platform holding water.
src:
Gdavid, replying to a direct question, Trawler Forum, 2026-07-23
Buying
CONTEXT
Loop popularity is both a resale strength and an entry premium
Gdavid rates resale as possibly the model's greatest strength: Loop popularity makes them easy to resell and holds their value — but you pay a slight premium at entry. Not a defect, a trade: buyers rarely price the premium in, and it is offset by exit liquidity.
src:
Gdavid, Trawler Forum, Jul 2026
CONTEXT
Do not read 430 reviews as 390 reviews
Rslifkin1 calls the 430 a different, faster, “basically planing” hull, distinct from the 350/390/400. Gdavid independently describes the 350/390 as semi-planing. Two independent sources agree the 430 is not the same animal, though a competing semi-displacement framing of the 430 still stands. Reviews and owner reports of the 430 are about a different boat.
src:
Rslifkin1 and Gdavid, Trawler Forum
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