diff --git a/src/components/interests/interest-form.tsx b/src/components/interests/interest-form.tsx
index 61612135..93b84557 100644
--- a/src/components/interests/interest-form.tsx
+++ b/src/components/interests/interest-form.tsx
@@ -860,6 +860,9 @@ interface DimensionInputProps {
   unit: 'ft' | 'm';
   ftValue: string | number | undefined;
   onChangeFt: (next: string | undefined) => void;
+  /** B3 #1: greyed out when the interest is set to read dimensions
+   *  off the linked yacht instead of these fields. */
+  disabled?: boolean;
 }
 
 /**
@@ -878,6 +881,7 @@ function DimensionInput({
   unit,
   ftValue,
   onChangeFt,
+  disabled,
 }: DimensionInputProps) {
   const focusedRef = useRef(false);
   const [display, setDisplay] = useState<string>(() => computeDisplay(ftValue, unit));
@@ -904,6 +908,7 @@ function DimensionInput({
         min={0}
         placeholder={placeholder}
         value={display}
+        disabled={disabled}
         onFocus={() => {
           focusedRef.current = true;
         }}
diff --git a/src/lib/db/migrations/0087_interest_use_yacht_dims.sql b/src/lib/db/migrations/0087_interest_use_yacht_dims.sql
new file mode 100644
index 00000000..a2a6f73d
--- /dev/null
+++ b/src/lib/db/migrations/0087_interest_use_yacht_dims.sql
@@ -0,0 +1,11 @@
+-- 0087_interest_use_yacht_dims.sql
+-- Adds a per-interest toggle so the berth recommender can read the
+-- linked yacht's dimensions instead of the rep-entered `desired_*`
+-- values. Defaults to false so existing data continues to use the
+-- desired-dims path the recommender has always used.
+--
+-- Per docs/superpowers/audits/alpha-uat-master.md Bucket 3 #1:
+-- "Interest dimensions: dual-source model".
+
+ALTER TABLE interests
+  ADD COLUMN IF NOT EXISTS use_yacht_dimensions boolean NOT NULL DEFAULT false;
diff --git a/src/lib/db/schema/interests.ts b/src/lib/db/schema/interests.ts
index 3134097a..80401fda 100644
--- a/src/lib/db/schema/interests.ts
+++ b/src/lib/db/schema/interests.ts
@@ -102,6 +102,12 @@ export const interests = pgTable(
     desiredLengthUnit: text('desired_length_unit').notNull().default('ft'),
     desiredWidthUnit: text('desired_width_unit').notNull().default('ft'),
     desiredDraftUnit: text('desired_draft_unit').notNull().default('ft'),
+    /** Per-interest source-of-truth toggle for the berth recommender.
+     *  When true AND a yacht is linked, the recommender uses the yacht's
+     *  length/width/draft instead of the rep-entered desired_* columns
+     *  on this row. Defaults false so existing data continues to use the
+     *  desired-dims path. Migration 0087. */
+    useYachtDimensions: boolean('use_yacht_dimensions').notNull().default(false),
     archivedAt: timestamp('archived_at', { withTimezone: true }),
     createdAt: timestamp('created_at', { withTimezone: true }).notNull().defaultNow(),
     updatedAt: timestamp('updated_at', { withTimezone: true }).notNull().defaultNow(),
diff --git a/src/lib/services/berth-recommender.service.ts b/src/lib/services/berth-recommender.service.ts
index bc5d2b84..5b685d56 100644
--- a/src/lib/services/berth-recommender.service.ts
+++ b/src/lib/services/berth-recommender.service.ts
@@ -34,6 +34,7 @@ import { and, eq, inArray, sql } from 'drizzle-orm';
 import { db } from '@/lib/db';
 import { systemSettings } from '@/lib/db/schema/system';
 import { interests } from '@/lib/db/schema/interests';
+import { yachts } from '@/lib/db/schema/yachts';
 import { CodedError } from '@/lib/errors';
 
 // ─── Settings ──────────────────────────────────────────────────────────────
@@ -380,6 +381,11 @@ interface InterestInput {
   desiredWidthFt: number | null;
   desiredDraftFt: number | null;
   portId: string;
+  /** Audit/debug aid — which dimension set the predicates were built
+   *  from. Set by `loadInterestInput` based on the dual-source toggle
+   *  + yacht link. Not consumed by SQL; surfaced in tests and trace
+   *  logs only. */
+  dimensionsSource: 'interest' | 'yacht';
 }
 
 async function loadInterestInput(interestId: string): Promise<InterestInput | null> {
@@ -389,6 +395,8 @@ async function loadInterestInput(interestId: string): Promise<InterestInput | nu
       desiredWidthFt: interests.desiredWidthFt,
       desiredDraftFt: interests.desiredDraftFt,
       portId: interests.portId,
+      useYachtDimensions: interests.useYachtDimensions,
+      yachtId: interests.yachtId,
     })
     .from(interests)
     .where(eq(interests.id, interestId))
@@ -399,11 +407,46 @@ async function loadInterestInput(interestId: string): Promise<InterestInput | nu
     const n = parseFloat(v);
     return Number.isFinite(n) ? n : null;
   };
+
+  // Dual-source resolution: when the toggle is on AND a yacht is
+  // linked AND that yacht carries dimension data, the recommender
+  // uses the yacht's dims. Falls back to the rep-entered desired*
+  // columns whenever any of those preconditions fail.
+  if (row.useYachtDimensions && row.yachtId) {
+    const [yachtRow] = await db
+      .select({
+        lengthFt: yachts.lengthFt,
+        widthFt: yachts.widthFt,
+        draftFt: yachts.draftFt,
+      })
+      .from(yachts)
+      .where(eq(yachts.id, row.yachtId))
+      .limit(1);
+    if (yachtRow) {
+      const yLen = toNum(yachtRow.lengthFt);
+      const yWid = toNum(yachtRow.widthFt);
+      const yDrft = toNum(yachtRow.draftFt);
+      // Only switch to yacht dims when at least one yacht measurement
+      // is present — otherwise we'd silently downgrade an interest
+      // with desired dims set to "no dims at all".
+      if (yLen !== null || yWid !== null || yDrft !== null) {
+        return {
+          desiredLengthFt: yLen,
+          desiredWidthFt: yWid,
+          desiredDraftFt: yDrft,
+          portId: row.portId,
+          dimensionsSource: 'yacht',
+        };
+      }
+    }
+  }
+
   return {
     desiredLengthFt: toNum(row.desiredLengthFt),
     desiredWidthFt: toNum(row.desiredWidthFt),
     desiredDraftFt: toNum(row.desiredDraftFt),
     portId: row.portId,
+    dimensionsSource: 'interest',
   };
 }
 
diff --git a/src/lib/validators/interests.ts b/src/lib/validators/interests.ts
index 0e396e02..c1dad9f3 100644
--- a/src/lib/validators/interests.ts
+++ b/src/lib/validators/interests.ts
@@ -84,6 +84,11 @@ export const createInterestSchema = z.object({
   desiredLengthUnit: desiredUnitSchema,
   desiredWidthUnit: desiredUnitSchema,
   desiredDraftUnit: desiredUnitSchema,
+  /** Toggle: when true and a yacht is linked, the berth recommender
+   *  reads the yacht's dimensions instead of the desired_* columns
+   *  above. Per migration 0087. Defaults false everywhere it isn't
+   *  explicitly set. */
+  useYachtDimensions: z.boolean().optional(),
 });
 
 // ─── Update ──────────────────────────────────────────────────────────────────